Almost all dispensing problems are the result of improper temperature, improper pressure, or cleaning/sanitation issues.
Reviewing this checklist should solve most common issues.
Is the keg being stored between 36-38 degrees F?
Is the CO2 pressure between 7-10 lbs?
Have the Faucet, beer line, and keg coupler been cleaned (with chemicals specifically manufactured for beer line cleaning) on a regular basis?
Some other common problems and their causes:
WILD BEER: Beer, when drawn, is all foam, or too much foam and not enough liquid beer:
Beer temperature is too warm
CO2 pressure is set too high
Faucet in bad, dirty, or worn condition
Kinks, twists or other obstructions in the beer line
Beer drawn improperly
FLAT BEER: Foamy head disappears quickly; beer lacks brewery fresh flavor:
Beer temperature is too cold
CO2 pressure is set too low
CLOUDY BEER: Beer in glass appears hazy, not clear:
Frozen or nearly frozen beer
Beer that has been un-refrigerated for long periods of time
Dirty faucet, beer line, and/or keg coupler
FALSE HEAD: Large soap-like bubbles, head dissolves very quickly: