There are things you should know about mastering. Mastering will clean a mix, but there are flaws that can’t be removed. There are things you didn't notice in the mix, but you'll notice them at mastering: most of them can be corrected. The mixes can be made brighter or heavier, the stereo width can be changed and the mix can be made to sound tighter and together.
If a mix needs to be redone, it should be redone; sometimes there are problems no mastering can take care of them. That's why it is recommended to send a preliminary mix to the mastering studio and ask for the mastering engineer's opinion; he can point to problems that can be and should be fixed in the mix.