Bad electrical connection between trailer and truck

The first thing I would be checking, is the cord a little too short to fit securely into the plug on the truck? When turning, maybe there is just enough bind on the connection to bend the blades without pulling out completely?

2nd thing I would check is the receptacle on truck to make sure the content points are not cracked or broken?

I tow a fiver. I am sure someone will have some better ideas for you. This is just a start I would check.