So, as you still haven't mentioned what type hitch you have this may be worthless info, but here goes.
I've a Reese Signature series hitch. The lever that has the hole in it for the pin to slide through is controlled by a tab on the inside and under the hitch clamp. When I first started hitching fivers I did what you just mentioned, raised and lowered the hitch after it was in place. This actually bent the tab completely out of the way so there was no way (other than with a long screwdriver) to get it to line up with the locking hole.
As previously mentioned, you want to insure the kingpin plate is lower than the hitch head when you start the process. (this, of course, after removing the little locking lever and straightening the bent tab) As your fiver rides up on the hitch it will be properly aligned to the tab and when the hitch closes the kingpin will have pushed the tab/locking lever into the proper location.
Sounds complicated, but looking under/through the hitch with a good flashlight will clear this all up. If your tab is bent (from raising and lowering the fiver after it is in a hitched position) there is no way for the king pin to push it into the proper locking position. And if this is not the issue, well then I've likely given all of you something to chuckle about and say, "yup, rookies"....
