Ha, good luck with that. These things don't get fixed. Having said that, I don't have any complaints about the B6A specifically. Its behavior seems consistent with the game. The small non-powered front wheels easily get stuck, and they would in real life too. Try feathering the throttle and avoiding obstacles like trees and rocks. There are also some 4WD tractors in the mod section. MTZ 82 is probably the best.
Here's one of my old posts on this subject:
"I don't mean to be contrary for any purpose other than to defend my best buddy tractor friend; that thing does not get stuck, but the front wheels have a bunch of trouble with rocks and logs. Either back up and drive the front wheels around the visible obstacle, or just wiggle the steering back and forth to get the front wheels to "swim" around it if you can't see it. Wiggling also get through deep mud when the front tires stall. Also key: don't spin the tires. Back off the throttle until the wheels barely turn, hold it there and wiggle the steering if needed.
Again, i don't want to start something, but the tractor doesn't hold you back, it makes you learn if you haven't already picked up these techiques."
Edit: haha, back from when i was still patient enough to be apologetic.