I've been riding my ATAC's for a few years and gone through several pairs of cleats. The most common problem is some creaking. I solved this by dousing the pedals in wax lube every few weeks (I've heard that this is what many people who ride ATAC's have to do).
I remember them feeling loose at some point in my history with them but I can't remember what I did to make it go away. The only thing I can think is that if they get too dirty they may not engage properly.
Other than that, check the security of your cleats and check to see if the pedal spindle is loose in the pedal.