We checked the nut at the bottom, and it was fine.

So, we took both the couplings off and examined them further.

The first thing we noticed was that the couplings appeared to be mounted slightly too low. So, when we put the motor housing back on, it may have been pushing the couplings down on the cylinder.

Here is what we did to patch/remedy the issue:

1: Used a dremel tool with a cutoff wheel to lightly score the cylinder (to create some friction for the couplings to adhere to)
2: Moved both coupling halves up slightly on the cylinder.
3: Reassembled and ran a job.

So far, the results are positive. There is no slippage of the couplings.

NOTE: We did originally try to wrap a tiny sheet of aluminum around the cylinder to use as friction, but the tolerances of the couplings to the cylinder was too tight. It would not fit. So, we came up with an alternative idea in scoring the cylinder to create friction.

Anyway... Thanks for the tips. The tip on the aluminum sheet is what mainly led us to success. (that idea eventually led us to consider creating friction using a different method)
Let's hope this stop-gap method works over time.