I want to change out an HOM 1-pole breaker with an HOMT 1-pole breaker.
(The HOMT is a tandem-type breaker that would add an additional circuit
breaker to accommodate an additional circuit in the single space.) However,
I cannot get the HOMT breaker to fully seat in the panel.

Is there any trick to getting the tandem breaker to seat? I have examined
the mating surfaces and I cannot see any difference in design that would
prevent full seating. I even tried plenty of force (which usually doesn't
let me down).

Any suggestions?

Thanks in advance.