Editorial Comment:  Without actually seeing the tires and the vehicle it is difficult to make an accurate diagnosis, however, based on the description I'll try to tell you where you might find some clues.

Since this is affecting only the front wheels and they seemed to be loosing pressure before and after changing tires I would start by seeing if there is anything that might be affecting only the front wheels and not the rest.

You could try putting these wheels on the back and see if they act differently in that location. If the other wheels from the back lose pressure on the front, you have a locational problem. If these wheels lose pressure in another location, you have a problem that goes with the wheels.

Is there anything unusual in the way the vehicle is parked? Are there any curbs or ridges that might put pressure on the tires at the front?

Have the front wheels ever been damaged through hitting a curb, a pot hole, or other kind of impact? Sometimes just a very minor distortion of a wheel, putting it out-of-round would be enough to provoke a slow leak.

I'd like to think that your tire valves were changed when you got new tires. They're really quite cheap but if they were defective or damaged and not changed, they might be the reason for the pressure loss.

These may not be the only causes, but they represent to ones which seem most likely given the description that you've provided.

TG