Problems with permissions

I'm having a problem with a site that I have exported and imported into another site. On the import I told the site to inherit permissions. Then I broke permissions and assigned unique permissions. 

The problem is, some of the users have full control but they are having problems editing the site. It is not like this always. At first we had a problem. Then the users could edit the site. Then again they could not after a couple of days. I fixed it today by changing he security group from full control to read permissions. Then i switched it back to full control. Now those users can edit the site. I don't think it's a permanent fix though. I suspect after a few days the problem will arise again.

Has anyone else had this problem. Any idea why it would do this? I have exported and imported sites before. I don't recall it having any problems with permissions.

Thanks

LSTalbot