good job on the strongid, many people never rotate wormers, then have so called "hard keepers' wait a month then hit her with a Quest gel,
Hard keepers tend to come down to a few obvious issues that seems like people want to ignore. Instead they use a shot gun approach of supplements, oils and doodads.
Hard keepers routinely come down to a few basic issues, High calorie can mask some of them but dont solve the problem
Not enough grass or decent hay,,, this can be behavior as well if a very submissive horse is in with others and not getting its share.
Worms, giving a ivermectin 1.87 over and over is not "Worming" chemicals need to be rotated and especially an occasional strongid or quest gel OR get a fecal count done.
bad teeth, if it aint chewed up it wont get digested and nutrients just get pooped out.
Ulcers, common in TB's
And rarely some kinda other medical condition.