When you expose the emulsion you won't necessarily see any image; it's only when you wash it out that it appears. When you wet the emulsion it may change colour but then the part not exposed will completely wash away leaving the exposed emulsion in the screen. If all the emulsion washes out then you have not exposed it long enough.
You need to:
Degrease the screen before coating and allow the screen to dry.
Scoop coat the emulsion.
Once coated, make sure the emulsion is completely dry before exposing.
Do a test strip to find out the ideal exposure time. (This may be where you have gone wrong.)
Don't use too much force when washing it out. A garden hose on spray is enough.