2012 Frontier carb model. Only 100 hours.

Trans issue.

Ive 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.
With engine off and in neutral and rolled the Argo quarter inch forward and it slips into H but not L. I move it another quarter inch and it will slip into L but not back to H.
What is going on?
I need to sort this out quickly to repair fences for cattle. Thanks for any help. I have no idea about trans.