I'm a Betty Crocker baker, so I am one that uses mixes all the time.
Are they baked all the way thru?
Do you cool them on a cooling rack so the steam can escape or do you cool them in the pan or on a cookie sheet, so the steam has nowhere to go but back inside the cake?
This may be overly obvious, but you are measuring the liquid properly, right?
For your future reference, here are two cake problem troubleshooting charts:
http://tinyurl.com/2p5bdu
(short list from joyofbaking.com.)
http://tinyurl.com/3dmp96
(long list from Sarah Phillips of baking911.com.)
HTH
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