I recently installed three Gerber Ultra Dual Flush toilets and each of them has a problem. These are pressure-assist models using an Ecoflush cartridge.

1) One of them came with a bowl that was 3/8" out of level at the back. I checked the floor and it is perfectly level, so the problem is entirely with the bowl. Upon the advice of a local plumber, I went ahead and installed it with doubled-up shims to bring it up to level. Is this likely to perform well and be reliable in the future? How did this bowl end up being so far out of level? This one is especially frustrating since I already replaced one defective bowl that sprayed water up and over the rim.

2) The second one releases air bubbles from the trapway each time I flush. It removes waste reasonably well (although not as well as expected) but it is obviously not performing as intended. There is not any excess wax at the flange. This one goes into a 2.5" flange insert and then into a 90 degree bend about six inches below the floor. Does the bubble indicate a vent blockage? Is this likely to cause any problems in the future?

3) The third one flushes the bulk of the waste, but leaves a little bit of backwash in the bowl (shreds of paper, tiny flecks of waste). The flush is a little bit sluggish and it creates some turbulence at the trapway entrance that may be causing the problem. The manual mentions that a weak flush could be caused by low water line pressure (not a problem) or by a clogged inlet filter (not a problem). Any ideas on what else could be causing this problem, and how to fix it?