I've had this problem on the last three screens I washed out. I've been exposing them for the same amount of time with the same glass as normal but:
A) It's been washing out way too much. It normally washes out great.
B) Each time I've washed it out, 'drips' have appeared from the 'top' (edge of the screen nearest the ceiling) which have washed out even quicker then the rest ruining the design.
Initially I thought it may be that the screen had been used a few times without being degreased so I gave it a good clean down, left it to dry etc etc. No difference. Then I tried a completely new screen and had the same problem so I'm guessing it isn't screen cleaner stuck in the mesh or anything. I'm totally confused now.