If you are using a mounting media such as cytoseal 60, it contains toluene which causes emulsions when mixed with water. Make sure you dehydrate thoroughly in 100% alcohol, and incorporate a clearing agent afterwards such as citri solv. try this. If it doesn't work immediately leave your slides to dry completely then try mounting. If you are somewhere really humid this can be tricky, you might have to turn up the air conditioning. Also, your mounting media may be contaminated, try using another labs or a fresh bottle if this solves the trick.