It's really common for them to hold a poop when something has changed -- like when they are in a new environment, or have travelled (similar to a lot of people, really!). With dogs, a day or two is fine to wait. Leo tends to be constipated for a day when he comes back to me from boarding. Jaspar is alwasy fine. If you want to move things along, give a teaspoon of cod liver oil or a tin of sardines. If it got to three days I'd call the vet just to make sure there are no other problems. Almost every foster I have had won;t go the first day they are here.