Where is return supply air coming from?

I just installed 12x6 air return grill above the bedroom door. Since my doors only have 1/2 inch opening om the bottom, I thought the 12x6 grill will increase air flow from the bedroom and it will make it cooler when doors are closed. Well, the draft through the grill is barely noticeable, I was expecting a bit more.
So, I assume the return air is coming from other rooms, but they have same 1/2 inch space on the bottom door. Checking by hand I only feel faint draft at the bottom, when doors are closed.

My AC is new 3T with 20x20x1 filter, in family room with windows and 3 bedroom doors. There is no return duct system, the return air supply is directly below the AC drawing ari from family room.

Is there way to discover where is the return air coming from? Garage doors and front doors seem to be working very well as nothing is coming through the sides, cracks, same for windows, I don't feel any wind coming in.

Is there way to discover where is the return air coming from?

John D.