I have a 12 row mounted 7100 planter that is really ticking me off.  Here is the problem:  I am shooting for a seed depth of about 2", sometimes it is right on target and sometimes Iam getting the seed only 1/4" deep.  This is on no-till wheat stubble that was anhydroused this spring and it's been pretty dry here as of late.  I know the seed bed is a little on the firm side, so I tried it in a field of standing corn stalks that is still very mellow and same result.  This appears to only be happening on the middle 6 rows, the wings dont seem to have this problem (at least that I can see or not as bad).  Have noticed that the gauge wheels are touching the ground but can be lifted up and spun easily. 

I have tried changing the angle of the toolbar/ parallel arms.  Also, I have taken basically all pressure off the press wheels. I do have 2-100 gallon tanks mounted on the middle section of the toolbar that I usually fill about 1/2 full of 10-34-0.  I have heavy duty down pressure spring kits from shoup, and in the middle 6 rows I have 60# bags of sand in the insectice boxes, in the outer wing rows I poured 80# of concrete in the insecticide boxes. 

 Prior to this season I replaced the parallel arm bushings.  The holes in the arms themselves were ovaled out so I had some bushings made then drilled the holes in the arms out.  I know some of them were a little tighter than I would have liked but figured they would wear in.  So about the only theory we have left is that some of the row units are sticking and holding others down (out of the ground).  Or maybe sticking up and not going in the ground.  But given the weight I have on the toolbar, and the downpressure this doesn't seem likely. And I did take a high lift jack and took them through the range of motion just as a double check, seemed to check out ok.  I plan to do this again just to cover the bases one more time

 I know there are some very knowledgeable people on here.  Hopefully you guys can give me some ideas, keep me from going nuts!

I should add that I have had this planter for about 5 seasons and have never had this problem before.

Thanks

Ryan