IF you have multiple rows of creatures in play, the row underneath the main row, but at the top of the "in play" box has a bad slot. Its the third slot from the right, if there is a creature in this slot you can target him but to no effect. I noticed this when i have alot of skeletons out there, and I can mutate all of them except for whichever poor soul is in this spot. Target him/yes, actually affect him/no.