An interior door in my house has a loose door knob. By loose, I mean the knob sometimes unscrews from the rod to which it was attached. If I screw it back on enough that this does not happen, then either the latch or knob won't turn.

I bought a replacement knob, which has the same problem.

What is the problem here? Is there a problem with my door rather than the knob? Am I installing the door knob wrong in some way? Some other issue?

Thanks.

 

Seems you have an old-timey mortise lock, and knobs coming loose was always the #1 problem, as the set screw in the inside knob would come loose. It was always a chore to keep it tight (before thread-locking compounds were used).

If the lock still doesn't work well after ensuring the new knob is well secured to the spindle, remove the knob/spindle assembly and, using a screwdriver or the spindle with a pair of pliers to operate it, see if the hub (the square hole in the lock) is turning 1/8 turn each way, and thereby retracting the latch. If it's not operating smoothly, you need to take it apart and clean & re-grease the parts that show wear, ie., the hub cams and latch guides. Look for broken springs as well. Nine times out of ten, re-lubing will restore smooth operation. the tenth time, the lock is broken, or otherwise "plumb wore out".

 

I should also mention that you should be troubleshooting the lock with the door open, so as to isolate lock problems from door/frame/strikeplate alignment problems, dealt with separately.

 

Remove the knob and make 100% sure the screw is able to screw far enough in to hit the flat on that shaft.
Anytime I've tried one of those cheap after market knobs the threads where so poor I had to run a tap into the hole to clean them up.
Make 100% sure the screw is sitting on the flat part of the shaft.
It's a little tricky adjusting that style knob, have to make a compromise, to tight it will not turn, slowly back it off 90 deg. to hit the next flat and try again.
A drop of Blue Loc Tite should keep that locking screw from backing out.