I have been running tubeless tires on Mavic Crossmax rims for the past year and rarely have to add any air. Are you running UST tires? Are you sure they are well seated on the rims? We have had to use an air compressor sometimes to get them to seat initially. You should hear a pop when they finally set the bead.

If you can put some Stan's in them you should be able to see where they are leaking. I find that one of the main benefits of using the tubeless tires is to be able to run much lower pressures, providing much better traction on the rough stuff. I run about 30 in mine.