polish

What type of rocks are you trying to polish? Mixing rocks of different hardness can result in poor polish.
The polish phase needs more cushioning. Various cushioning materials are suggested on different sites: clean plastic pellets, cruched walnut shells, cut up rubber bands (the neutral tan color, not dyed colors which might leach dye during the tumble and stain the rocks), cornmeal and more.
Cleaning every speck of grit from previous stages is necessary to get the stones to polish, especially if there are any cracks or pits that might have trapped some grit. Some books recommend sorting the stones before the polish stage: only use the totally smooth ones in the polish and start any others again at the first stage with a new batch. Some books recommend a wash stage before polishing, tumble the stones with a thick slurry of Ivory Snow flakes. Using a toothbrush to scrub stones is suggested on www.roughandtumble.com/tumblinginstructions/. Another site that emphasizes thorough cleaning is www.gemworld.com/Tumbling.asp.
Several instructions say to add either sugar or corn syrup to the polish phase. This makes a thicker slurry which will slow down the tumbling action for a better polish. The tumbling info on www.rocks4u.com/tumblers.htm advises to add twice as much sugar as polish.
Now the warning, all of the above hints come from written books or web sites. I am new to tumbling and haven't actually tumbled my first batch yet. But as you can see, I've been doing a whole lot of research. Maybe someone with more experience will be able to better pinpoint what your proble might be. Please let me know what ends up working best for you.
Ellen in LA, aka mudd1973