" is there anything special going on with bigger sprites ?"
did you try reducing the size of the sprite?
I doubt that's it.
I don't know how crowded and cluttered your capx is but when I run into a problem I can't easily
figure out, I open up a fresh capx and recreate the 'problem code' to isolate it, and see if it happens again.
maybe do a test on a fresh capx?