Roller door problem

I have a shed that has a roller door on it, with an automatic opener. It is much closer to one side than the other, so much so that it is quite uneven and noisy as it goes up and down. One side will not go down at all, until finally it goes with a quick slide down to match the other side, and a rumble. Eventually to will open/close all the way though. The side rails can be moved in-out a bit so I have moved out the side that catches a few times. Each time it is nice and smooth and quiet for a bit (and I can see a few mm gap between the door and rail) and then it starts all over again. I am sure that the door must be not bolted down quite properly (there is just a u-bolt on each end of the central axle) and it must be shifting. I am thinking of moving the whole door sideways a bit and then tightening down the u-bolts. What is the easiest way to achieve that without throwing out the opener settings too much. I imagine the door is going to be heavy. Do I loosen the bolts off and then try and lever it? Any ideas? Thanks

Just be aware that there is a coil spring under tension, probably inside the roller.
Not prepared to comment further without images.

NO! do not undo the u-bolts. It might look like a simple mechanism, but if you undo those bolts you will be in a world of trouble. It sounds more like the door has become out of level somehow, but if you know nothing about roller doors you would be well advised to call someone who specialises in them.

You need to consult a door installer to fix the doors operation. What ever it costs is saving any injury to yourself
These things could be lethal.

What the others have said, get a door installer to fix, DO NOT ATTEMPT IT YOURSELF