I'm not sure that your emulsion is the problem. Do you know what mesh count your screen is? The higher the mesh count the more detail you'll get out of the screen.
This thread has some information on mesh counts.http://www.t-shirtforums.com/screen-printing/t113401.html
You can also go to http://www.rhinotoughgraphics.com
to get some screens at a higher mesh count for a little less money than the local art stores.