Sometimes if you can't get a gear, push the propulsion lever forward and use the inching peddle to stop drive. It makes it easier to mesh the gears in or out. This may not be your problem though. Try it anyway.
Get it fixed. My little 1440 never went in third quite right I thought, but had no trouble...until it jumped out of gear going down a 1/2 mile blacktop hill on our place this year. I got it stopped with out dropping the corn head by standing on the brakes, but would not recomend doing it daily. Used up a lot of hill doing it. Thank God I did not meet any of the normal traffic, or I would have to run it over the side, and it is a long way down. ( they pick up speed fast. )