I'm wondering if the seat clamp ears and lugs might have stretched from overtigntening as previously mentioned. Once the Al material is stretched, there's no bending back that will cure the problem as might be done on steel frames, but you might be able to file off some material between the ears and the slot the get back the tightness needed to hold the seatpost properly, just watch out for any cracks that might be developing at the clamp/lug area as the Al might have been stretched and stressed to its limit already. If you see any, drop everything and check out the possibilty of using and external, add-on calmp instead, as already recommended.
If the lug and clamp area seems to be sound, you might also be able to use some sort of split steel or Al shimming tube too, instead of filing material off.

Chombi