We're sorry to hear about the trouble getting the PaintMate working properly. Some thick paints may have trouble getting through the roller cover, for those materials there are a few things you can do to paint/stain to ensure it applies well.
1 - Stir the paint/stain with a mixer in a drill until its creamy smooth, about 1 min per gallon
2 - Strain the paint/stain before it goes into the roller
3 - Add a flow agent, like Floetrol or Paint Easy (this is not a thinner) for latex paint – Penetrol for oil based
4 - Or you can thin, even as little as 5% can make a big difference, water for water based materials and mineral spirit for oil based material.