The promo packs you get at events tend to have troubles importing.

I'm not sure if the problem is with Deckbox, Moxfield, or the MTV CSV Helper tool I use. However, When I export from Moxfield for example:

Moxfield Export: Assemble the Players, pmkm, 3p
Deckbox Export: Assemble the Players, Murders at Karlov Manor Promo Pack, PP_MKM 3
CSV Helper Moxfield to Deckbox Export: Assemble the players, Murders at Karlov Manor Promos, 3p

This is just one example. But nearly all promo pack cards if not all have this problem currently.