Rear prop shaft bolt keeps coming lose

I am having an issue here. I noticed the the bolt on the rea prop shaft coming out of the tranny was gone. It actually broke off. I drilled and easy outed out but i found out that it was not even tight. I replaced the seal aling with new bolt and washer. Lock tighted with blue. Couple days later, bolt was gone again. Got new bolt, red lock tighted it couple days later, gone again. Another bolt and added lock washer and next day see it lose again. What’s happening here? Crappy lick tights? I will try it again soon when i get new lock tight. Any other suggestions? I should add that last summer i replaced the u joints on that shaft, but been good till now.