It might be stuck and if it is, it could be pretty easy to free up or could be impossible to free up or anywhere in between. Penetrating oil such as PB Blaster and patience is probably your best chance of getting it moving unless you can subcontract the job to a neighbor/friend/relative.
At the risk of coming across as condescending, which is not my intent at all, might be good to first make sure you’re using the correct procedure to adjust the seat, particularly since you said you have a LA765 tractor (a sign your tractor fu may be a bit weak) and stated it “may be stuck” (a sign you aren’t sure it’s stuck).
Unlike some others here, I can’t list off all the tractors that use the LA765 loader. I know the L4701 does because I have one. So, first step: Check what model tractor you have. In the below pic, the loader is a LA765. The tractor is a L4701. So check the hood for the model number. If there’s nothing there, check the badge on the right frame rail just behind the front tire.
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If you have a L4701, the below may be helpful. If you have something else, maybe not. If you have something else, check the manual to be sure you’re following correct procedure.
Below is an excerpt from the Owner Manual for the L4701 explaining how to slide the seat forward/backward.
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This is a picture of the actual release lever.
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Sitting in the seat, reach down just behind and below your right knee to find the lever and rotate it up. Then scoot your butt to slide the seat forward or back.
In case that doesn’t all make complete sense, this is how you move the lever to release the seat slide. I couldn’t move the seat and photo it while in the seat because I’m not a contortionist, but here it is anyway.
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