[Robert Blanker]

Took some time when i had the same problem. I dont use a tablet, just normal mouse drawing.

Anyway, to fix the problem;
1. Select the layer in layer listing.
2. Look at the ‘lock:’ option on layer listing at the top, its below the drop downlist with ‘Normal’, ‘Dissolve’ etc. and Opacity dropdown. The first option after ‘lock:’ is Lock transparent pixels. Make sure its not activated. Should solve your problem…