Won?t go into High or Low?

2012 Frontier carb model. Only 100 hours.

Trans issue.

I?ve just noticed an issue when changing gears. I stop, shift into neutral to idle or reverse. When I try to put back into high or low range it won?t go in. I have to put into reverse to move a little then it will engage. It only happened yesterday and became more frequent during the day.
Today started and won?t go in high or low but will reverse. Putting in R and moving then tried putting in H or L but will not engage.
It does not crunch/grind while trying to engage H or L.
When engine cold oil on dipstick is full. Oil is clear.
Engine idle is 900rpm.
Thoughts?