There is probably debris in there, the motor senses the resistance and stops.
My suggestions:
- just don't mess with it. If the glass is attached and not leaking, you can drive the car. I was in this situation a year ago to the day, kept messing with it, and ended up with a convertible x5 in the dead of winter. If you put your hand up in the middle it deflects a lot of the wind. Wear good gloves and switch hands. Above 50mph was not possible with all the wind.
- since your tilt works, tilt it open and try to spray compressed air in there. You could also use a little bit of a spray grease of sorts at this point.
- you could then (at your own risk) use the allen key tool in the tool kit to try and manually crank it back. You might want to do a visual inspection of both sides first and make sure nothing is obviously out of place. There are so many small cheap parts that can be broken, and if you crank it, more will break, and then it won't close. Then try to get it to close, and it all snaps apart and the glass is halfway off the car.
So if you feel any resistance or hear any bad noises (apart from the drain channel noises that are normal) stop, do not continue to try and crank as I did. If you can get it to open, properly clean and grease.
- tilt up and remove the front and rear glass panels, fully clean and grease. This is somewhat of a smarter option. If you don't have any idea how bad it is (i.e. just got the car), this would definately be smarter.
The R/R isn't as bad as you'd think, the hardest part for me was reinstalling the headliner, there are 3 clips on the rear that you have to get to slide over at the same time. This was near impossible and almost reqires a second hand. Once you get the casette out it's just a screwdriver to change the parts out. There is a good video. Getting the casette up and down isn't bad, raise the seats/headrests, and a jackstand on a box works nicely to support the rear.