moving Emails

I have a user that has a issue i have not seen before. He can’t move some emails from his inbox to his PSTs. I thought it was that the PST was too full so i created a new one, but still emails over 2 MB or so don’t transfer out of the Inbox. Has anybody heard of this before? Or is there something i am overlooking? Emails under 2MB seem to work. I could be wrong with the size, but i think its really close to that. Im sure ive missed some info that you might want, so ask away