I think I have found the cause of my problems. My heated pad dips down in the middle, so that when the extruder tried to lay down filament where it thinks Z = 0.2mm is, the bed is actually at Z = -0.5mm. The filament has cooled before it hist the bed. I can fix the problem by propping up the middle of the pad by putting some 1/4/ x 1/4 balsawood sticks between the carriage plate and the pad.

I tried the trick of laying down a wash of PVA and water on the glass. It seems to provide good adhesion.

OME