Not sure if it's a similar setup to my 84 trx 200, but mine sometimes gives me trouble switching beteween high, low, and reverse. I've had good luck shifting into first gear while messing with the lever and it usually clicks in. And +1 on pulling up on the lever (if you're not already doing it).
Jason
85 ATC 250R - restored stock other than 18" rears, nerf bars, Honda key switch, 14T front sprocket, and white tank and plastics (except rad shrouds)
87 ATC 200X - restored stock other than 18" rears and nerf bars
84 TRX 200 - rough but complete budget restore for wife and eventually daughters to bum around on
Eton Viper 50cc - oldest daughter's current ride