Drawer Slides

Sounds like the drawer rails are not quite perpendicular to the face of the cabinet. Adjust the rear mounts away from the protruding corner, e.g. if the left front corner is sticking out a bit, move the rear mounts slightly to the right, and vice versa.

Unless the drawer is exceptionally wide or exceptionally narrow, you'll want to move the rear mounts by about half the distance that the front corner sticks out. For wide drawers, more than half; for narrow drawers, less.

While the drawer is square, the cabinet might not be, eh?

In any event, adjust the position of the drawer slide on the offending side toward the back of the cabinet. If you're lucky, that should take care of it.

Sounds like the cabinet sides my not be perfectly parallel. A quick fix assuming the cabinet is not too far out of square is to stick a wooden shim or a washer behind the offending end of the drawer slide. If you're worried about the washer or shim falling out, then put it where the drawer slide is screwed into place.

Sounds like the cabinet side of the slide is extending out farther that the other side. While they may appear to be the same distance from the front of the cabinet, one side of the cabinet may protrude out farther than the other side.