Think you skipped a word there ;)
They probably just became unthrifty and eventually died. We aren't keeping birds in the wild, we're putting groups of them together in varying sizes of enclosures and possibly free range areas. They're going to pick up more worms. Sure, you can let them just...
I kinda didn't notice the wormed monthly part. Personally, I wouldn't do that... I tend to do it once in the middle of winter when they're slacking on eggs anyway, unless I get someone new.
If you're using the correct dosage and not under-dosing, and you're following up with the second dose in 10 days like you're supposed to, that should minimize resistance. Otherwise, I think the only way you'll know for sure is if you take before and after fecals and see if it's getting them all.
I still wish we could buy Valbazen in smaller bottles. The kennel site that used to sell it doesn't anymore. I found some tablets and liquid on eBay, but they're from Malaysia and Korea, just not worth it to me.