I've had this problem a couple of times too, though on the German side. In my case it was part of the 'running out of oxygen while surfaced' problem, but I've never heared of a US sub having this problem.
Try saving and reloading, for me that helped.
Battle stations could be the problem too, but you say you've secured from battle stations. It is possible your crew is too tired to work, but if you wait for about half a day your crew should be rested.
Anyway, if this problem is somehow caused by your crew it should be noticeable on the crew tab. Anything weird there, like all crew sleeping?
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