I believe this machine has tapered pins that hold the bobtach in place.
Are you using a impact wrench to tighten the bolts that hold the pins in place???
If you are not using an impact wrench then that may be one of the problems.
Another issue is if the pin and the bushing are worn out, it will have a very hard time staying tight.
Another issue is grease. If grease gets in the threads of the bolt that holds the pin it will have a hard time staying tight.
So how do you get the pin out??? Remove the bolt until there is some thread left to hold it in place. Then push the bolt inward and this will push the pin ou of the loader arm and into the bobtach. Inspect the pin and bushing for wear. If they are worn, then replace them.
How to reinstall, remove all grease and oil from pin and threads and bolt threads. Stick pin back into bobtach so that it clears the loader arm. Then add semi perminent loctite to bolt threads. Stick bolt through the lift arm and thread the bolt into the pin. Make sure no grease or oil get onto the new or clean parts. Then use an impact wrench to tighten it up. For extra security I then use a breaker bar to get the bolt extra tight.