[Lynn Eades]

Pull it to bits, and clean it up. Then you can hopefully eliminate it as the problem.
When someone recons one of these boxes, it may mean nothing happened to the top of the box.
As you can see, there are three holes in line with the three selector shafts. Above each shaft is a ball brg. followed by a spring. On the opposite plane are two pellets between the three shafts. these are interlocks, which in conjuction with a floating pin in the center shaft, prevent you from selecting two gears at once.
At one end there are core plugs (or frost, or welch) which can hold a fair bit of gunge.
Its worth the effort to go through it. Pull it to bits slowly, taking careful note of how it all fits together (your photos will help a lot.
The shaft with no fork is for reverse. The one at the front of the box is for third and top gear, and the other one selects first and second. If you have any problems, I can walk you through it.
There are other areas that can cause your trouble too, as George said. If your clutch is dragging it makes selecting a gear harder to do. If this is the problem, it will be noticeably different when the engine is running, and not running.
Any questions, Just ask. It may help someone else.