Jim,

Just for experimentation, I would try setting it higher to see if that makes a difference. You would not have to print the complete part, just up to where it starts doing infill and see what happens. It sounds to me like you extruded filament is not sticking to the lower level. At 20%, there is not a lot to stick to? I mentioned temperature also because with a 20% infill, there is not much heat retention. Certainly less than with solid infill.