Sounds like it could be a root problem. If you watered the tree enough during the winter it should have developed new roots. If your soil is very heavy clay though, it may not have been able to grow enough roots to sustain the tree.

Or, If the tree is sitting in a "clay bowl" of a planting hole it could be drowning. I lost a cherry tree that way. We kept watering but it just looked worse and worse every day. Then we noticed that the water was sitting on the ground around the trunk of the tree and wasn't draining away.

I have also seen trees (and other plants) droop when there is a lot of wind. Wind dries out the tree and the soil very quickly, and the roots can't provide adequate moisture. The wind this past week has wilted some of my well established plants, but they will recover if this wind ever stops blowing, or if it rains today like it's supposed to. If you get rain today and your tree perks up, even temporarily, I'd chalk it up to the wind and leave the roots alone for now.

If you don't see any signs of disease or parasites on the leaves and bark of the tree, that's good. Younger trees usually adapt more quickly than older ones. I like to start them when they are very small and usually have to wait 5-6 years before they fruit, but it's worth the wait.

Then, of course, sometimes the tree just doesn't 'take'. We lost a nice young oak tree last year, in spite of all the loving care we lavished on it. I snapped off the trunk and it was dead as a hammer. When I dug up the root ball, all the new roots looked strong. Just didn't seem to be any reason for it to die, so who knows? Cheryl