Help! Lil Mynx mount for static pole

I am finally putting up the LM mount.  I had a guy use a stud finder...sawdust was on the drill bit so he hit a stud. Now, Im screwing the lag bolt up into the  mount and hole, I have the mount flush against the ceiling. I only tighten until I feel resistance but when i take my hand off of the mount, it isnt up against the ceiling anymore, it actually can spin and is quite loose. So... when I take the screw out to start over, there is white drywall dust on it. Did i strip it? I only tightned until it was secure....