Rear wheel won't move or keeps falling off
I'm going to say that you have the axle washer on the wrong side, should be inside the dropout rather than on the outside, doing so can stress the disc brake as it was aligned with it (the washer) on the other side, or your simply pushing the frame into the rotational surface so much that it can't move due to the pressure on the entire surface rather than just the axle.
Basing my answer on experience and reading through this



When it's in the wrong spot ( on the outside ) you are also pushing the frame against the surface of the hub which when tightened enough will prevent it from being able to turn. Washers prevent this by adding both space and reducing friction between a moving object and a static one. Essentially a bearing turns fine on an axle but if you sandwich something up against the entire surface of the bearing it wont move due to the friction present on the entire surface area rather than just the axle sleeve.