Loader arm doesn't want to engage tractor on one side

It would surprise me you bent the loader unhooking or re-hooking. It would be my guess you are not setting on level ground or not coming into it straight on. Be sure the mounts on the tractor are not loose either.

I always find loader re connections go better with two people.... one to move the hydraulics and the other person to install pins.

Dave

I've been attaching / detaching my loader for a year and never had a problem, until last week, when the same thing happened to me: right side went on, left side wouldn't. I can see why you thought you bent the arm, because I thought mine was bent too: it even looks bent when it gets crooked like that.
VERY FRUSTRATING! I couldn't get the dang thing on, and couldn't get it off. Finally, I was able to get it "close enough" to "on" that I could get a 1/2" socket handle though the side that wasn't going on, just to help line it up to take the pinned side off. Once off, I backed up and drove into it again, and it went on no problem. I do a little side work with my tractor, and that dang FEL made me late for a job.
Yesterday I took it off, realized I took it off in the wrong location in the barn, put it back on, moved it, and took it off again, and nope, not bent.

I've had three different Kubotas (Kubotae?) with loaders starting in 2002. Unless there's a need for the loader, it's not on the tractor. I've had days when the loader was on and off more than once. It's a very quick and simple thing to do. Almost without exception the swap is in the shed or on the blacktop. I never understood why anyone had difficulty.

UNTIL

One day last Summer I dropped the loader on an uneven gravel drive and had one awful time getting it back on. It wasn't the first time the loader was removed there, but that particular spot must have been a bit more "out of spec" for the operation. After slightly missing the connection on the first try, the loader shifted a bit. It was hard to believe how difficult it was to do something that had been so easy had all of a sudden become so hard. It wasn't nearly close enough to do what tawood did. I ended up doing a bit of curl, then using chunks of 2x4's under the lip of the bucket and dumping it to press against the wood to compensate for the uneven terrain.

Unless the loader uprights have a bit of play in them (comes with age and use), the loader must be sitting level and the tractor must also be level to the uprights when installing. I take mine off in the grass but I put wood planks under the unloading feet when I take it off or it becomes a frustrating process to get it back on which has become less as the fulcrum pins in the upright joints get more slop in them from use.

I tend to leave my loaders on all the time and just remove whatever I have attached to the quick detach.