If your print is too light, not enough ink is passing through the image area of the screen to the print surface. Check to see that your image has properly developed on the screen. If the screen has been overexposed, there may be photo emulsion stuck in the image area. Any area of the screen that prints should have open screen mesh, rather than photo emulsion in it. Solve this problem by exposing the image on another screen with a shorter exposure time.
If your print is too light, and you are sure your image area is open, try thinning your ink with solvent or reducer until the squeegee can force the ink through the screen. If you do not want to thin the ink, try making a new screen with a more open screen mesh.
If your image blurs, too much ink is being forced through the image area of the screen with the squeegee. Check the image area of the screen to make sure that the screen was not underexposed. An underexposed screen with have open mesh outside the image area. Try making a new screen and increasing your exposure time.
If your image blurs and you are sure that only the image area has open mesh, try exposing the image on a new screen on a finer mesh.
If part or all of your screen double-prints in some areas---one image right next to the other---this is called ghosting. This is the result of screen mesh that is not taut in the screen frame. The screen moves during printing, resulting in ghosting. Try re-stretching the screen mesh over the frame, taut as the head of a drum. You should be able to bounce a coin on it. Most screen printing suppliers sell pre-stretched screens.
You may have trouble with multi-color jobs printing out of registration. Carefully register the job on the press before production. Print the keyline color, usually black, with registration marks showing. Register the other colors to this key color, lining up the registration marks. Print a proof of the multi color job on practice paper or fabric before committing to expensive T-shirts or paper. Tape over the registration marks before beginning production. If the job loses registration during your press run, check to see that the screen clamps are tight and in good working order. Check the registration notch or tab to see that it is true. Re-register the out-of-registration colors and proof the job again before continuing production.
If you experience screen breakdown during production, your screen may be old. Make a new screen with the same design. Your screen may have been made with emulsion that is beyond its effective shelf life. Mix a new batch of photo emulsion and make a new screen. Your ink may be incompatable with your photo emulsion. Use ink and photo emulsion that are manufactured to work together.
In printing T-shirts or fabric, the shirt may come off the shirt board between colors, ruining the print. Use more spray adhesive or other shirt board adhesive to hold the shirt down between colors.
Small dots of ink, called pinholes, appear during printing. Cover the pinholes with a small bit of tape on the back side of the screen and continue printing. If many pinholes appear, the screen is breaking down and should be replaced.