Does not build pressure past 20 PSI

Hello, My compressor does not build pressure over twenty PSI. the engine keeps running and does not shut off. i bought it brand new in 2010 and used it very lightly, less than five hours of engine run time total. i changed the oil only once, after the break in period. Thanks, Juro Davisburg, MI p.s. This is the compressor http://www.harborfreight.com/air-tools/air-compressors/21-gal-2-12-hp-125-psi-cast-iron-vertical-air-compressor-67847.html#pr-header-back-to-top-link