Okay, I feel like an idiot. I borrowed a friend's new Hitachi NR90AC3 framing nailer, bought some paslode nails from Home Depot & fired the gun one time into a test piece of 2x4. The nailer stripped 5 nails from the strip, & promptly jammed.
So I cleared out the nails only to find the plunger (right word?) still depressed. I didn't realize the Hitachi gun required plastic collated nails (the paslodes were paper collated).
I called Hitachi who told me to "put a screwdriver against the plunger & tap it back down with a hammer", which I did.
Now the nailer works - but this was a scary learning experience (I was afraid I had just bought my friend a new nailer). Bottom line: make sure you know what nails to put into the nailer if you ever borrow one of these!
Anyway, it now fires nails - but I see a spark whenever it drives a nail - which I only hope is normal.