Did you use the full dose recommended for full size horses? That little container treats a 1200 lb horse, the smaller your mini, the less you'd use. It's really hard to measure, too.
I have a friend that has used Bio Spot for horses since it first came out - on her two arabs and my two ponies that she leased for 2 years - and won't go with out it. She takes the dogs to the vet to be treated (and bathed etc), but does her horses herself.
Most of mine, I had no problems with when I used it but with more than 30 head right now, it's more than I want to do.
Had a boarded 1/2 welsh/arab that had a severe reaction to regular fly spray (we use the permethrin concentrate and mix it with water) whom the vet recommended we not try it on - took forever to find a fly spray that worked for him and I can't remember now what was used on that one... When the owner moved him to her own property, she continued using the one we found that worked for him.
Currently we mix our own fly spray (as above), use fly masks on the ones who need them, use fly traps around the barn and mow/spread the paddocks/pastures. I also use a homemade "Swat" - the fly spray we use mixed with Vaseline to use inside of ears around face/muzzle and on bellies and chests on the ones that need it (one of our arabs and on anyone riding/driving out in the woods).
Really - how fly sprays/products work seems to vary with the horses, the location (state, city, county) and the weather. We've used fly spray that a friend 5 miles one direction swears by and it won't work for 2 minutes on any of our horses and the same with another friend who lives 6.5 miles the other direction. In the same token - neither will use the fly spray that I mix up - says it creates reactions or doesn't work at all. Some work with some horses and some will not.
My farrier makes a natural, home made brew that is water based. Other than it needs to be re-applied constantly, it's great!! I purchase a bottle every so often from her and that is what we use when she comes to do hooves every two weeks.