Broken lock pin is my guess..
You should see the little metal tab thing move out and snap back in when you put the bar in.
Then, you should be able to swivel the bar back and forth and it not fall out.
Of course, if you swivel it far enough, that's when it disengages the pin and you can take the bar out.
Or, you have a small rock stuck up inside the hole and the bar can't seat right and won't engage the pin..
I've had that happen before when I drug my hitch head thur some gravel on my steep driveway and got one of them stuck..
Anyway, it's all about those pins. It should be easy to replace it. Just have to get a new one and maybe carry a spare one with you..
I've been running for over 15 years with the same Eaz-Lift setup and have not had that issue yet!.. I keep telling myself "I should get a spare bar pin..", but still have not done it!
Good luck!
Mitch
2013 F150 XLT 4x4 SuperCab Max Tow Egoboost 3.73 gears #7700 GVWR #1920 payload. 2019 Rockwood Mini Lite 2511S.