I have Emtek and there is a setscrew, as fixer said, which requires the right size hex wrench. Hopefully, yours hasn't completely fallen out and disappeared. However, if your particular doorknob does not utilize a setscrew, it may have a hidden spring and catch. Such doorknobs must be fully seated on the stem by pushing on the knob fairly forcefully (you may hear a slight snap when it seats). If the doorknob is not fully seated, the catch won't be engaged.

Set screw came loose. Refastenen it.

Thanks for your advice. I don't see the set screw. It may have gone out the front door when the door was open. I guess I will have to call Emtek to find out the size of the screw to replace it, or maybe they will send it to me. I have a set of Milwaukee allen wrenches (actually two, for U.S. and Metric measurements), so I should have the right size wrench, but the set screw itself is a mystery.

Kudzu9 - I tried what you said, but I don't think that is the case with my particular model knob.

a little loctite 242 will stop it from falling out again.

friedajune-

I have gotten free parts from Emtek customer service in the past for one of my locksets so, hopefully, they can send you a set screw. However, you should also be able to take the knob to any decent hardware store and get one of the right size for a few cents.

A new one will cost you $0.29 at the hardware store and you will be done in 10 minutes. Not worth the effort to get one from the manufacturer.

Hardware stores do not have sets screws with points. Only shaft set screws.

Grind it down

fixer-

1. My Emtek doorknobs employ standard set screws.

2. My local, old-fashioned hardware store carries both kinds. (Not sure if Home Depot does.)

I found the set screw in a corner. It's small so it was practically invisible on the floor (floor is travertine with a lot of pattern). The allen wrench that fit was the 3/32 for an idea of how tiny it is. The knob is now solidly on the spindle and works great. Thanks everyone.

Great that you found it before it got vacuumed up. You might want to go around to the other knobs with your Allen wrench and make sure the set screws are all well-seated.