Since this is a "home" mastering job, are we talking about nearfields in a so-so room? If so, does the guy doing the "mastering" slouch? Doesn't take much off-center with nearfields before things shift.
S of M's comment "Also, most DIY mastering job are done with less than adequate monitoring and acoustics where you could be doing more damage than good. " - This is definitely the norm. After reading about several mastering rooms/engineers, I've almost given up on being able to afford a facility that should even attempt serious mastering. Hard enough to get rooms that work well for tracking and mixing. Hell, these guys spend more on a pair of speakers than I've spent on my entire studio, and I've been at it a while. About $75k so far...
Were you listening to the mastered versions in the same environment they were mixed? Have you checked several commercial recordings there? There's just a lot of things that can go wrong here. So far, everything that's been mentioned could be the problem, or just some, or a few hundred that haven't even been mentioned.
You need to use a scientific approach - S of M's other comment "Check your mix on a couple different speaker systems and check your mix in mono to make sure its the mastering environment thats off and not the mix" is good advice - do this with some commercial recordings too, everywhere you check your mix. If your monitor system can't make a commercial CD sound right, then your mix won't be right and it can only go downhill from there - mastering trying to compensate for a bad mix, etc.
All in all, some good advice from everyone - good hunting... Steve