Underexposing the emulsion is the most likely culprit in why it is washing out. Check to see if the emulsion on the squeegee side gets sticky or slimy as you are rinsing out the screen. Some may even notice a quicker run-off on that side or a sudsy, soap-like appearance. These are clear indicators that the emulsion is underexposed. Try adding a minute or two to your exposure time or switch to a faster exposing emulsion.
Be careful not to use excessive water pressure as this can also tear parts of unexposed emulsion.