Rawhide expands and contracts all the time naturally. It's really apparent during times of high humidity, like when a thunderstorm comes through the area. If they are exposed to severe humidity like in a box, in a truck or warehouse, they actually go flat but they will tighten up as the hides adjust.
Cowhide which is very thick takes a little longer to dry out than deer, elk, or buffalo drums. Big pow wow drums will take more time because the slats are more sealed than smaller hand drums. Tree trunk log drums take the longest because the wood can hold moisture too. The air inside has to adjust too so that can take a while. Basically the drums are severely traumatized by the humid shipping experience. But, once they dry and tighten the affect of the change of weather will not be as extreme.
The drums will be great again once they dry. Rawhide can do some strange things on drums because of humidity. The hides get wavy, bubbled, sunken and of course sounds flat. Occasionally, people that are unfamiliar with rawhide drums will return drums without telling us, saying they are dead. But it is so dry in El Paso that by the time we receive the return and open the box the drum is perfectly tight and sounds great.
Usually if you just keep the drums in an air conditioned house they start to dry out real well. Keep them on their sides so both hides are exposed to the air. You can also set them outside on their sides in direct sunlight for short periods of time, rotating the opposite side to the sun, if it is not too humid out. You also don't want to over tighten them, that is a worse situation because the hides can split along the edges.
The moisture in the hide is the issue, as long as it takes to dry inside and out is when they will be tight again and that is when they will sound good. With small drums we recommend using a hair dryer on hot to tighten the hide quickly. The Indians hold their drums in front of a fire for a few minutes to dry them. A fire really dries the air so it works well.
But with big drums, the hide is so thick that it takes a while to dry inside. For example, if you put the drum in the sun, the hides will begin to tighten but when you remove it from the sun, it goes flat again. This is because the air inside is still humid. If you repeat the process a few times and keep it in the air conditioning, it will get tight and sound good. In extreme cases we re-wet the hides until they are soft again and they will re-dry tight.
Take some time and dry them out, you will see what I'm talking about with the way the hides react. You will have your drums in good shape and sounding great, very soon.