truss movement causing cracks

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Now I understand you post better. Thanks for looking at the drawing. Lets see....Your house was build during the winter. At that cooler wetter time of year, the trusses actually contract, pulling the bottom chord up. In summer, the joists expand again, and push the ceiling down. This is a phenomenon known as truss lift. Drywall ceilings are often allowed to float along fixed interior walls so that the expansion and contraction is absorbed in the tape, or is not transmitted to the wall. This is done by resting the ceiling drywall panel on the wall, but not nailing to the bottom chord of the truss.

In your case, the trusses actually pull to a normal level in winter, but impinge on the wall in summer. The fix of shimming the trusses up may relieve this, but will have to be in combination with repair and proper attachment of the drywall. The idea is to let the chords of the trusses travel with seasonal changes without causing movement of the drywall along walls under the truss.

There are lots of articles on the internet that discuss truss lift and drywall attachment strategies. Do a search and read a few of the articles. It will help you discuss the problem with the engineer and builder. If you have questions after reading, post back, and I'll try to help. I have relatively little experience working with drywall in truss construction but have seen similar problems. If you have any problems finding material on truss lift and drywall, I can give you some sites.