How old? My DS switched daycare centers twice (once at 2, the second time at 4), and it took him a good amount of time to adjust to each. The second time (at 4) it was a good month before he adjusted to the new center. But after that month, he loved it and when I'd come to pick him up in the afternoon, he'd push me away and say, "go back to work, Mommy!"
It'll happen. But I know it's so hard until then! Hang in there.