welcome nontavitch!
I've been using the UV10 all year with great success.
A couple things, vibratory tumblers should not start with "coarse" rock generally. These tumblers will polish most hard rock, but if it starts coarse they don't do as much to "round off" the rock, so rock can stay deeply pitted and not look all nice and round. Usually rock is first rounded from weeks in a rotary tumbler, work on a wheel to smooth it, or is a cabochon or slab preform that is essentially already the shape that you want it. If you put in something really coarse, it won't get all nice and round, it will just get shiny unless you have other issues...
The pitting, in general, can be a big cause of the problem. First, it carries grit from earlier stages to finer stages, and keeps rocks from ever getting shiny. This is the cardinal rule of tumbling - your grit can't contaminate later stages or your work is in vain.
undercutting can carry grit as well - if little cracks form the grit won't rinse out and comes out in a finer stage and contaminates it.
The other thing that makes rocks look bad in polish when they seem to be getting better is that the polish is very small and fits into little cracks. This means that if you have material that tends to crack or undercut (uneven hardness usually) that the polish will color all the little fissures white and look bad.
Here is my process, adapted from Randy's here:
forum.rocktumblinghobby.com/index.cgi?board=tottumb&action=display&thread=293691) start with as nice a material as you can, not all cruddy and fractured unless you don't have anything nice. Make sure your rocks are similar hardness (poke around for help on hardness if you don't know) - avoid rock that has mixed hardness as it will undercut.
2) smooth and round off your rocks, either on a wheel, in a rotary tumbler or start with very smooth stones (no pits or cracks). If you have a rotary and need help with this step (usually called stage 1, the coarse tumble, or 60/90 or 46/70) just ask, maybe on a new thread.
3) CLEAN EVERYTHING METICULOUSLY after the rotary tumbler and don't let rocks that have pits or cracks move forward, put them back on the wheel or in the coarse tumble.
4) fill the UV10 right to the top of the center volcano thing with rock. Flush with it, no higher and shouldn't be less. Use wet stones, and make sure there are a lot of small (1/2" and less) stones, like 50% small stones or ceramic media - this keeps things in contact with other things better and avoids breakage as well as keeps grit on all the surfaces.
5) turn it on - it will be loud.
6) add 4 tb 120/220 Silicon Carbide grit into the stones, and they will quiet down. Don't just dump it all in, sort of pour it around and let it spread out and make sure things keep moving nicely.
7) seal it up tight and let it go an hour or so
8) open it up and make sure things are moving nicely. If they seem to be slowing down spray water in. As fwfranklin said you will need to check this stage more since it will generally remove more material and make a thicker slurry. If you see it slowing down, spray it with a water bottle 4-5 times. If it thickens up, spray it and I use my hand to sort of stir things up until everything is loose and moving agian. You want to avoid getting it too sloppy though because then the grit gets rinsed off the rocks and can wear out your bowl. You should plan on checking this stage 3-4 times a day. Keep this going ** 2 days **
9) CLEAN EVERYTHING METICULOUSLY AND REPEAT STEPS 4-6 EXACTLY, WITH 120/220 GRIT THEN MOVE ON TO STEP 10
10) CLEAN EVERYTHING METICULOUSLY, REPEAT STEPS 4-8 except with 500 Aluminum Oxide, let it run 2 days. You can let this run longer for prettier stones. The Aluminum Oxide will break down eventually into a decent polish. But I like mine crazy shiny so After as many days as I feel like waiting (probably no more than 5 days) I move on to Step 11
11) CLEAN EVERYTHING METICULOUSLY - but not in the same collander or sieve, or classifier or whatever as you used for the earlier stage or for your rotary tumble. I have a dedicated screen for cleaning things after the AO500 step. I wash everything thoroughly, if there are things with cracks or pits I put them aside for another batch.
12) remove the bown from the UV10 and thoroughly clean the threaded post, washer and wing nut so no grit or slurry gets carried into the polish stage.
13) REPLACE THE BOWN WITH A DIFFERENT BOWL THAN YOU USED FOR THE OTHER STEPS - yep, you heard right, don't reuse the same bowl. I have a dedicated bowl for polishing. Nothing that was used for any other step comes anywhere close to my polish stuff. I keep the AO polish in a tupperware with the polish in bags and dedicated measuring spoons. I close it as soon as I am done.
14) Put the rocks in the new dedicated bowl like you did for the other steps
15) turn on the vibe
16) slowly add 4tb of AO polish so it doesn't glue up
17) let it run 3-5 days, checking periodically. I find that the polish stage and even the AO500 stage tend to not get too thick but you should keep an eye on it. Eventually you can hear how it is going and tell when it is too thick.
18) clean everything thoroughly and put the rocks back into the bowl
19) I add 1/3 bar grated ivory soap (I bought like a 32-bar pack off amazon with free shipping cheap so I use that) into the bowl with the wet rocks and run it a couple hours. At this stage I am just cleaning any residue from polish off the rocks, this makes them super-duper shiny if you haven't caused any other issues along the way by contaminating.
20) Rinse everything and put on a clean surface to dry. I like to use a clean t-shirt on the kitchen counter. I try to pull the rock out by hand and shake it off so I get the water off as I go, and lay the tshirt across my lap as a sort of satchel to put the rocks in, then carry them into the house to dry and so I can look at them.