Are you familiar with sanding in an X type pattern.
What are you using to sand with? It's best to use sanding block but by block sanding wrapping sand paper around a Paint Stir stick works well also. You can also use your hand if you can keep it flat.
I couldn't find any example pictures but what you do is say start at Top left and for your sanding stroke go to the bottom right, then start from the top right and for your sanding stroke go to the bottom left. Keep alternating as you get comfortable the technique does take some practice.
What tools, and body filler are you using?