This weekend I finished running in my TM928, all went great, eventually started running 80-90% full, and for the first 2 fuel tanks i was as happy as can be, then I started staling, at least once every lap maybe two laps, the track no it all told me I need to add spacers in my motor and that the compression is to high, my concern is that I ran 2 fuel tanks through it without this problem, how sure can i be that this is the actual problem?

I dunno but I try to always start with the cheaper and easiest solution first. I always have extra pre-oiled filters I prepare in a medicine container. You would be surprised what a difference a clean air filter makes compared to a dirty one. I wouldn't be saying this if I didn't experience it. Just try it before you start adding spacers to places that don't need to be spaced.

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.

Thanks guys; ill do as you recommend a complete once over, I did try to change pugs when this was happening. Just for information im running a 25% fuel