you may still need to loosen the pedals on the MTB. If you aren't having any problems with your road bike - I'm guessing the tension on the MTB pedals may be too tight. I keep my MTB pedals Reallll loose coz I suck at MTB'ng and I fall at least once a trip - even with loose clips! LOL You might try loosening the screw alll the way and then tightening down say 1/2 turn or 1/4 turn and try it.
Let us know how it works out.