OK, if another matching prop color does the same thing, there is likely something wrong with the plastic clip mechanism with that specific motor. I suggest you look at it very closely with a magnifying glass, comparing to the motor on the opposite site, to see what might be broken, bent or simply molded wrong, out of tolerance for mechanical fit. It may be the case where the plastic is simply out of tolerance and that's why it doesn't attached smoothly and securely like the others. You can buy replacements for those plastic mounting latches
here for $9/pair.
Is this a brand new craft that hasn't flown yet? If yes, did you buy it at a local store where you can exchange the craft, such as Best Buy or Frys?