I programmed a key for a beltpack (BP) on my panel and when I call the BP, I can't latch the key.  

When you configure a new Beltpack Role in EHX, the Latch Disable feature is checked by default.  This prevents other panels / beltpacks calling this role to be able to latch their talk key. 

To change this, in EHX, go to Beltpack Roles - Global Options and uncheck the Latch Disable feature.