I own two acoustic guitars. One has a very noticeable bubble under the bridge and I believe the other is beginning to bubble as well. Can anyone tell tell me what could be causing this? The fact that it is happening to both of them means it is probably something I am doing wrong. Could I be stringing them incorrectly? I have heard that heat will do it, but that can't be the issue. I haven't had either of them in any extreme heat (or cold for that matter). Please help. Thank you.