something that's randomly popped up from time to time in pve spars (and from some personal experience, in actual pvp) is that after a spar, you won't use your shield at all until you leave world and log back in.
First log i have of it happening is a spar with ammar during the impalong, i didn't block once at all during the spar, but i blocked in my previous spar just a few minutes ago. After relogging i could block just fine, and we had a rematch.
Second one is with 2 spar today with emoo, where he could block with his shield in the first spar with madnath, but then couldn't block with his shield in the second spar with me.