The HyperDeck Mini doesn't officially support H.264 for playing back externally encoded clips. There is a note about this in the manual, and some additional discussion in these threads:
viewtopic.php?f=4&t=77493
viewtopic.php?f=4&t=81351
Your best bet if encoding clips for playback on the HyperDeck Mini is to use ProRes or DNxHD.