Do a simple run down before adding head shims or taken shims out. Try another glow plug, new air filter, check fuel lines for air leaks along with the fuel filter. Im guessing this is a new engine and your working on break in? If so, make sure the pinch bolt gets tightened, thats the bolt that holds the carb in place. I see more folks forget to check those and cause tuning troubles due to an air leak.
The rb 928 can run 20% or 30% without adding another head shim, most .21 and above engines are set for 30% nitro and if your running 20% then adding a shim is pointless. If your running 20% try 30% same brand as the 20%.
Lots of info on this board for running down engine issues, these are just a few common ones, check them first and good luck.