Using the largest possible jetway doesn't automatically ensure it will reach a door. In fact, sometimes it cannot reach a door *because* it's too long.
Which is why, GSX has a "Test" feature in the parking editor ( NumPad 5 when a jetway is selected ), so you won't have to wonder if the jetway you chose will work with a particular airplane: if you use that airplane while customizing, the test function will make a sound if at least one jetway will work, with a text saying which jetways could reach which doors.