Tony is right. You have to create air circulation. Moving the cold air return to floor level is the easiest as it is only one intake rather than a lot of vents.

For the inlaws I just used some flexible dryer vent hose and ran it down an unfinished wall then stuck a intake grill at the bottom. This was a good way to test the best placement of the intake, for the best circulation. The basement is now finished and the dryer hose is now hidden.

I personally have to add another return in my basement because of the L shape. The one intake does not have enough draw for the whole basement.

Thanks Fellows!
I will keep this in mind when I start to develope my basement soon