The unfortunate truth for tech people is that moving jobs is the most effective way to get a promotion. If you're hiring for a company that will quickly and effectively "promote" people who perform well, I wouldn't think you'd have any problems.
If employees are going to outgrow their positions with you, though, and don't have a position to grow into, then you should dig. :)
>Moving on wasn't completely your choice.
I'd think that a personality "flaw" isn't completely someone's choice, either.
Of course, digging hard also reveals a potential "personality" flaw on the part of the hiring company. As mrkurt suggests, if you're having trouble with retention, examine and improve yourself first.
There's a lot of good reasons for people moving around, but I have to make absolutely certain that it's not a personality flaw that is in the candidate.
Note that if you've spent time as a contractor and have indicated that you're working on 12 - 24 month contracts, that's entirely different. Moving on wasn't completely your choice.