Chickens have worms. Worming merely decreases the load. Anything that stresses the bird environmentally or systemically can cause their immune systems to get behind the load faster.
I presume you are rotating your fields (if possible) and picking up litter. As you can imagine, chickens peck and eat everything on the ground including another's poo, and thereby ingest the worms. So field rotation, litter rotation, as well as wormer rotation can help with loads.
You've used safeguard (fenbendazole) and valbazen (albendazole) repeated times as a standby....it may not be as effective on your worm loads if the worms were not knocked out by the first round, then you changed wormers to another wormer med that would have had a different efficacy...you may have inadvertently harvested the strongest worms against those meds by not following the protocol for either effectively.
If you change wormers, I would follow the protocol with one wormer only. Then, next time follow protocol with another wormer only.
Usually it is good to not throw everything at an animal, espeically if you only do half protocol, as you begin to limit your options of effectiveness with over exposure.
I would use the Wazine again as full protocol and see what happens.
If you still have heavy worm load, I would try Ivermectin (there are those who swear against Ivermectin as ineffective, but numerous studies show it can be a good choice for the right situation and I have personally had very good results with it.) If you go Ivermectin cattle pour on, it only takes a couple of drops at the base of the neck and below the vent (being careful to never get any in the vent), but to be effective it is essential to treat 3 times, 7-10 days apart. Studies show Ivermectin removes at 50%, then about 80% then after third does 99%.
So you can see what I mean by the importance of following up appropriate dosage with the appropriate wormer...otherwise you are simply growing worm resistance with a half effort.
After that, it is a matter of keeping stress free, happy hens. If you aren't doing so, I recommend apple cider vinegar in the water (raw with mother, plastic containers only) and regular probiotics either in feed additive or yogurt in mash treat. Neither are de-wormers, but they can help with gut flora which strengthens the immune system, the bird's first defense against worms.
Pumpkin seeds and cayenne pepper act as worm flushes, but you have to keep pristine field rotation or you simply flush for the next bird to eat.
Edited by Lady of McCamley - 11/22/15 at 6:51pm