Metal Bottle

Sounds like the female foster fitting has a problem. It should not come off, even under high pressure, unless the locking/grasping mechanism is faulty.

Yes, it is usually the locking mechanism. Grab the collar firmly once you have pushed things into place tightly and try to push it on all they way you should feel it click into place. If it won't go you check the o-ring inside the female fitting for problems otherwise you have an incompatibility issue and one of the fittings will need replaced. It's not unusual for the Chinese fittings not to lock on correctly even units from the same vendor do not always lock together. I usually replace offending units with the Ninja brand units as they always fit together.

Had the same problem with a new fill hose one time. Turned out there was some metal shavings preventing the balls from seating properly. So yeah, check the female side for any debris.