Reports are very confused but the main problem is the sheer number of battle droids that the Feds can just throw away. The Fed tanks and artillery droids and the like are taking a toll on Selenians that cannot be replaced. The Selenian's own such firepower is only blowing away replaceable droids.
Aside from the high number of droids landing at the bridgerhead, as is the nature of a droid army, it is now clear that the Cult is performing all the 'securing' of the settlements inside the zone, freeing up the droid armies to attack.