Its a little different than an M4 stock, it has the little cutouts and it slides back and forth instead of the M4s90 which rotates to the side and slides.
First off, check to make sure there is nothing in the little groove cutouts, then when you're adjusting it, you can let go of the button a little before and let it slide into place. The Supernova/MR1 stocks are definitely a little rough to start off, so when you let go of the lock button and then slide the stock it should snap into place when it hits the cutout.
If you're not getting the correct LOP its always possible to get a gunsmith to make a new notch into the stock, which is what I did with my M4s90 stock, between the mid and fully collapsed position (for cold weather shooting with heavy clothes even the mid was a bit too long)