Where do you feel the looseness?
Is the neck isn't moving at all at the joint where it meets the body when you press on the fretboard with your hand?
Maybe just a cheap neck.....
There really isn't any visible separation but, it shouldn't bend and stay..it should bend and return...am I right or wrong?
Are you sure you aren't just pressing too hard and bending the strings? ........................................
If you're not seeing any cracks in the finish around the neck joint or any movement there, then you most likely have a poorly routed truss rod cavity. If there was room for the nut at either end to shift and then it stuck there until forced back, that might produce the symptom you're talking about. Try loosening the truss rod until there is no tension on it and see if you can repeat the problem. If it still does it, it's not the truss rod.
Why not? ......................