I have the same issue, two weeks ago I received two MiPads *64GB, one in green and one in yellow. The green one have no issues at all, the capacitive buttons are very responsive just as my mi3, but the yellow one, I have to keep my finger until it is recognized... I'm dissapointed about the QA apply to the MiPads.
I have not find any solution yet ... (I don't want to return it yet).