I have a PSE Shaman recurve 30#@28amo with a B55 flemish twist string. The string is about 4 months old. I shoot 100 arrows 3-4 times a week. When I check brace height before shooting it is 8". After shooting 100 arrows the brace height is 7 1/2". This often happens. Question is why?

I assume you are unstringing the bow in between uses?  I have friends who use dacron strings and they do shrink a little when unstrung, and stretch a little when strung.  My friends typically press down on the limb tips with the bow resting in their laps before shooting the bow to get the initial stretch into the string before they start shooting.  Measure your brace height after the string has stretched.

Too many plys and/ or too many twists?