I want to take a different course. "What if" the motor had been lying around and was old(er)? I've seen a bit of surface yutz develop on APCP grains that (OMG!) I mixed myself and have been sitting around for years. Put a "hot" igniter in 'em and they work just fine.
Now most of us have heard about the "Crapperhead" igniters of old. The two sided copper foiled thingies that AT provided in the past and I don't know if they still do it. Well, I was told by an old sage to take a razor knife lift off one side of the copper foil off the center insulator and use well supported clips or bare wire ends on them. I had no problemo with 'em most of the time.
Oh, in my now gone prefecture the wires that went to the pads had bare ends on them. One would wrap the igniter on the end of the wire and go. If using a "Copperhead" igniter, the flier knew they had to provide micro clip extensions to wrap on the bare wire ends leading to the ignition relay. Ummm, we also had wire cutters and wire strippers out at the pad if the ends got too dirty!
Getting back to the "Crapperheads", when I did as our prefect told that is peal the foil off one side, fully support the wires to the igniter and viola', I can't say they were any worse than any commercial igniter I've ever used............. Except my home brewed ones I think are better.
I do remember many of my rockets H and above to L sometimes taking their time getting off the pad with AT or my home grown igniters the formula(s) of which I pimped off the internet and mind you forget where I got the formulas.
I'm convinced that some oxidation occurs (ok slam me on this if you must) with "older motor grains" and it takes higher igniter heat to get them to "catch". A "hotter than Hades" igniter that burns longer can be very helpful in that case.
Getting back to the OP's post. It was said that the motor was replaced with a different one and I will venture a guess it might not have been as old as the first one with oxidative yutz on it. Hence it fired easily. Comments always welcomed.
There are three ways an igniter can be bad. 1) It can be shorted, which will draw a huge amount of current but not light the pyrogen. I got a batch of the non-Hazmat J's from Aerotech awhile ago, and every single igniter shipped with them was bad in this way. You should be able to find this situation with a DVM. 2) It can be open, and has essentially infinite resistance; it won't light the pyrogen either, obviously. You can also find this with a DVM; it will show as "open". 3) You can have a bad pyrogen dip, and the wire will heat up but not light the pyrogen (or the motor). Similarly, if you have an igniter designed for low-voltage and/or low-current, it is possible to overload the bridge wire if you put a significantly high enough voltage/current, it can cause the bridgewire to burn out so quickly that it doesn't transfer enough heat to the pyrogen to light it.
There's no easy non-destructive way to test for that, unfortunately; you have to try to light it after the fact and see if it lights. Your best pre-launch igniter test is to check the resistance with a DVM, if it's in the neighborhood of between 0.5 and 2.0 ohms it's probably good, depending on the type of igniter. If it's very near zero or open, it's probably bad.