The first few times I ran our party through combat I had a similar issue.
What fixed this was me reading up on all the abilities of the NPC's and putting them on paper. After that the NPC's got more interesting and challenging.
I will say "forgetting about an NPC's ability in the heat of the moment" has been my number one barrier to tense and exciting fights.
Another recent revelation to me was the way critical hits and injuries work. I've been afraid of assigning NPCs high-damage attacks, in the event that a crit comes up, I don't feel afraid to activate it. But then, reading the crit hit/injury rules, it sort of occurred to me that it's designed with this in mind. Anything that would take down a medium or larger target in one shot can automatically fall into the category of hits that can be used as injuries.