81 DR 500 Shifting Problem

I have a old 81 DR 500 and I love it! I live on Vancouver Island, BC. There is lots of hill climbing......

Recently I have developed a strange shifting issue. If the bike is in 1st, then i can get 2nd. But if shift into 3rd, I am now stuck with only 2nd and 3rd. I can't go back to first. If I upshift to 4th, then I'm stuck in 3rd and 4th and cannot go back down to 2nd.

I do get back down, intermitently, eventually. But it's not there when I want it.

Basically I get a 2 gear range out of 5 at any one time. So to finish my last ride just kept in the 2nd and 3rd gear range to get home. If I upshift, I'm stuck.

I hope I've explained this ok. Thanks in advance.

I got it just today. Pulled off the little cover that houses the shifting assembly. Turns out that where the Shift Cam makes contact with the Fork from the Shifter Shaft they weren't making proper contact. I had bent it on a rock recently I guess. Straightened it out and I'm back to all my gears.

Thanks