Presuming you have enough hides and your temperatures are in the right range (88–92 degrees F at the warm end, 75-80 on the cool end, with hides at or near both ends), and you're not trying to handle the gecko a lot yet (give him/her a while to get used to the new place before you start harrassing him/her
), there are still a couple of factors that are possible.
Sometimes mine goes off her food for a little while (a day or two beyond when she'd normally eat) after shedding, and obviously the stress of moving can put them off their food for a bit too. A thing to try would be to drop a cricket or two in there. Some geckos have to be trained that mealworms are food. Crickets, on the other hand, set off that reptilian 'if it moves and it's smaller than me, it must be food' instinct, and so are more readily seen as food by most lizards.
As far as how long this can go on without worrying, I've heard of people seeing them not eat for up to two weeks after a move to a new environment, but that would be an extreme example. Bottom line though, their tails store fat, so they can live for a fairly long time w/o food if they have a nice chubby tail. When you see the tail start to shrink, start worrying.
I'm sure others will have comments too, but I think I covered at least the basic possibilities here.
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0.1 Albino Leopard Gecko - Tigger
0.1 Crested Gecko - Pooh-Bear