I believe Frankie & RnR have identified the issue and given you good advice.
However... I don't want to sound like a dick, but this is the same advice Simon offered you HERE about sanding flat before applying the grain filler and using mist coats of spray paint to identify low spots.
I would also suggest keeping all these posts in your build diary so they're all in one place for future reference (for yourself as well as others) and it avoids us all jumping around trying to find things in order to help you (or others).