No, having an outbreak on antivirals isn't all that unusual. It does stop them entirely for some people, and in others, it just reduces them. It's still possible to get outbreaks while on them.
I wouldn't worry about resistance, or the medication not working any longer. Maybe you have a trigger that you don't know about? Is it cold where you are, or maybe you're eating something that's triggering it? Have you been sick - the flu or a cold? Really stressed or fatigued?
Triggers are different for everyone, and sometimes you just get an outbreak and never figure out why. You've been doing really well on the famciclovir, so I wouldn't expect that to change. Herpes doesn't become resistant to antivirals like other things become resistant to antibiotics and the like.
If you continue to get outbreaks, look for triggers. Think outside of the box, too - it could be as simple as a new lip balm or a food, but could also be another medication you might have just started using, a soap, shaving cream (if you're male), etc. Antivirals help a lot, but they can't override everything. I've had hsv2 for 15+ years, and acyclovir works wonderfully, but if I get a cold or the flu, forget it.
Hope you feel better soon!