Kubota M9000 Not moving in High Gear

[TexanRider]

Hello Everyone, I have a Kubota 2000 Model M9000 2WD with ROP and recently have a problem with it not moving while in High gear range. I bought the tractor last June with 335 hours on it. Previous owner didn't use it much, mostly for hay operation (loading and unloading round bales but not actual bailing or cutting).

I will give everyone a background prior to tractor having this problem. I took the tractor on a drive on road for total of 12 miles for diesel (Saturady). Everything was fine, took around 30 minutes. Got back on property and used it, no problem at all. Next morning used it again, no problem again. Following evening, got on it and tried moving it, the tractor would not move in High gear range. messed with it for short time and it moves in Low gear. It goes back and forward with no probem in Low gear. FEL and PTO works just fine. I checked Hydraulic Oil level and it was low or mostly outside the acceptable range.

I left it alone for a week, had to go out of town for work so couldn't look at it. This weekend I added Hydraulic oil to it (5 gallons, dip stick shows more than necessary). I checked what I can and still doesn't want to go. Don't see anything physically broken. I confirmed this morning and Low gear works as before (slow). I appreciate any help/suggestion anyone can give. Thanks. T.I.