Shouldn’t need to bolt the shed down to get the doors level IME.
Photo’s would help but I suspect that the sides aren’t plumb and therefore the door opening isn’t square, this will result in your doors being at different heights. Open the doors and accurately measure the aperture corner to corner, this is a more accurate way of checking than using a level. This will tell you if the opening is out of square/not plumb.
If the base is level and assuming your spirit level is true, then the sides of the shed are probably out of plumb and might need pushing over to square things up. Can you place shims between the house & shed?
Additionally, have you put your bikes and stuff in the shed yet? Once your shed is full of gear it might change how it is presently sitting.