It might be as simple as clutch dust getting in the gears. Its most common cause is draggings and pushing the gear ranges. But can happen over normal wear. When the clutch dust builds up, some people can't hardly go into gear just as you described. One more thing that's not really a fix, it's just sustaining the problem, give her some gas as you try going into gear. That should make it a little easier to slide into gear.
