Why won't Craftsman pressure washer won't go into high pressure?

This is difficult to know without a regular diagnostic procedure. Some problems are more likely with specific models. The most common Craftsman units useGenerac pumps. The Generac pumps that are older than 5-6 years old are very often not repairable. They used a pump head design that is nearly impossible to dissemble after a few years of corrosion...and they no longer offer the head assembly as a part...and it was often more costly than anyone wanted to pay for anyway. This pump can be recognized by it's six sided, finned housing. The pump head can be brass or aluminum, with a waffle design on the front. A small set screw looking nut is on top of the head outlet. Sears does use a few other pumps...most rather recently. See some of my other answers for typical pressure problems. Check the Sears parts store to identify your specific model and parts. www3.sears.com