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.
How old is the light source in your exposure unit? Same emulsion? Same number of coats? Same cure time? Can you post pics? It's hard to say w/out seeing what's happening, but all things being the same, it sounds like you might be underexposing your screens. Could be so many things. Might be time to replace your light(s) also.
I should have thought of pictures while washing out - unfortunately it hardly shows once dried. The lines are coming down the screen like water when it's washing out (as in, fast and in a constant stream).
Same emulsion, same number of coats and cure time. The light source has only been used for maybe 6 hours maximum so I'd be surprised if thats the issue. It's a small diy set up as I live in tiny rented accommodation but everything has been exactly the same. I might try simply blitzing the exposure next time and see if it makes any difference. Could it be an issue with the emulsion itself?
The emulsion has probably run it's course. Should usually try to use it up sooner than that especially if it's being stored at room temperature. Storing it in a refrigerator helps extend it's life some but 4 months is still pretty old. I would try some new emulsion before scratching your exposure time.
It could be a bunch of things.. old emulsion can burn a good looking screen, but then breaks down on the press. Try new stuff, also try hardening the screen after you burn it by putting it in the sun or on exposure unit.