Agree, she needs something more broad based and Valbazen is the go to here for a heavy worm load. The ivermectin should have gotten some out which is a help, now hit it with the strong stuff. No, the wormer should not lose much strength, unless it was open and not kept in ideal conditions....sometimes our barn gets hot and I think it decreases the effectiveness..... but she needs something stronger. Also, until you do it, give her any/all "probiotics/vitamins/any health building things. Stop the day before you dose her, and then wait 36-48 hours for her to pass them and then give her another round of whatever you are using to rebuild her strength.