I usually don't have any problem loosing the RTK signal.  The problem is that I loose the GPS satellites.  We have tall trees around many of our fields.  Running the headlands on autopilot is almost impossible on some of them.  I have some fields where I have to start on the line 2 or 3 passes from the edge and work back toward the trees with the marker down so that I can work the first pass in with the markers if necessary.

I have found that lowering the elevation mask helps some, but I am told that reduces the accuracy also.