As said above, the beading might not be sealed properly. Take it to a different tyre bay and have them re-fit the tyre and see weither that makes any difference. A decent garage would see the state of the inside of the rim and give it a clean with a wire brush, to make sure the tyre has a snug fit, to prevent any breaks within the seal. It could be a hair-line crack within your rim that the air is escaping out.
Hope it gets sorted mate.
p.s. as a last attempt to sort the problem out, you could always use, tyre weld, just spray it in the air valve.