Sounds like your water level could be too high, how far from the top is the water level?
Is it a custom made sump? I assume by your question that you have baffles installed.
Typically the return area is slightly lower because it's pulling the water out and the time for water to make it through the baffles takes a little longer but if not it's really not an issue for it to be level unless you have too much water in the sump. More then likely the baffle openings are a little too large or your not pumping very much through your sump if the water level is 2/3rds full.
How much flow do you have going through the sump? Is there a separate fuge? Can you post a photo?