make sure your fighting enemy's thatc on decent challenge (green) or higher. If you have trouble defeating higher level enemies, then group or duo them.
Yeah I did. It was green. Barely survive after killing 1 of those firefly. I thought it was weird when i got nothing when I killed 1. Rest, full HP, killed another firefly. Still no xp. But I got xp when I switch to another lancer and bow at rank 1.