Is there a way to check air flow patterns for the vents in a single story house? My masterbedroom is the only room not cooling sufficiently. Its about 6-8 degreees warmer than the rest of the house during the summer months than the rest of the house. The house is 2350 sq ft. with only one a/c unit, but two returns, one of which is a smaller return in the ceiling of the masterbedroom doorway. The refrigerant has been checked and is fine.

Any suggestions would be greatly appreciated.

You can use smoke tubes to show air flow. With 1 supply and 2 returns you may be short circuiting the air flow. Where are the supply and returns located?