Did you use Mike's skeleton for the bones of your shirt? If so, that may be the problem. It's a neat feature when in a rush but it's not a clean way of making a character in my opinion.

Take your clothing item back into the setup room (no Mike, just your clothes in the scene).  Then delete from the toes on up (if he has toes, if not, start at the feet). Leave the hip bone alone and all the bones that are actually used by the shirt. Save the file out under a new name. Test the shirt under the conditions of the original problem. Should be resolved I would think. 

In the future, you may want to try just making the shirt and creating bones with the propper internal names for the figure's parts instead of using the pre-existing bone set. I personally find that avoids problems like this but that's just me.

Now, if that's not the issue - if you did not use Mike's rig for your shirt - or if for some reason the resolution I provided didn't work, let us know. When you do, explain - STEP.BY.STEP. what you did within Poser to make the shirt a figure in the first place.  I may not be able to help but someone else may if they know what you did. Explaining your exact steps (or as close as you can) is the best way to get help to problems in software. 

Additionally, HBorre, is right - it's best to Pose the figure first and then to conform the clothing.