There are two goals with exposure.
Crosslink the emulsion with sensitizer so it is
durable and stays in the mesh. Crosslink all the sensitizer so when you want to
reclaim the mesh and remove the stencil, it falls off the mesh with little effort because stencil remover attacks the crosslinks.
But if you expose long enough to do all that crosslinking, you may also have too much UV light undercutting your positive and bouncing around in the mesh so that your fines lines close up and ink won't transfer through the mesh.
It's not really over exposure that is the problem, it's that exposure can change the size of your image. You can prevent this with faster exposing stencils, stronger, single point, calibrated metal halide lamps so the work of exposure is done quickly, before the light scatter gets you!
Here is one thread about this we have posted in the past:
The longer the exposure time the better?
Search this forum for light scatter and over exposure, or just plain exposure.
The screen is your printing tool, and learning to JUDGE exposure is the most important skill you need.