I sharpen the chain at 30 degrees and it doesn't cut well. I've lowered the guides almost to the stamped mark and they still don't cut well.
Welcome to A.S.!
I would not lower your depth gauges any more, just yet; you can always do that later if needed, but you can't put any more back on.
I don't know if you do any woodworking, but it is helpful to think of each cutter as a chisel or plane. Except that each cutter has 2 cutting edges; one on the top plate, and one on the side plate, as the illustrations from Oregon and STIHL (below) show.
Like a chisel, the profile angle (file guide angle in the top illustration) is an important part of the cutter's shape and performance, but you can have a 30 degree cutter profile and still have a dull edge. Both the top plate cutting edge and the side plate cutting edge need to be sharp, again like chisels.
The top plate edge separates the wood fibers when cross cutting and the side plate edge severs the fibers. If only one edge is sharp, you will experience poor cutting performance. Like on a woodworking plane, the depth gauges control how deep the cutter digs with each pass: too shallow and you don't take much wood; too deep and the teeth grab rather than cut.
Check to see if both edges are uniformly sharp on each cutter. If they are not, look at the way that the grinding wheel touches each edge. If you are not using the right size (thickness) wheel for your chain, if the profile/edge of the wheel is not dressed (half-round radius), if the head tilt on your grinder is not correct (typically 60 degrees) or if it is not grinding right up to the edge, you will not get sharp cutting edges on both the top and side plates.
Also visit the STIHL website and watch the video 'Chapter 3: Sharpening your Saw Chain'. This is for hand filing, but it may help you understand what you are doing with your grinder.
http://stihldealer.net/videolibrary/
Philbert