I haven't had much luck with Bronco, but I'm sure the flies are different everywhere you go. But in my experience, the cheaper the fly spray, the poorer it works. Manna Pro has been practically useless for me too.
I like "Pyranha Wipe N' Spray" the best of the ones I've tried. I have also heard Endure is good, but I have never tried it myself. After trying tons of fly sprays, including herbals (because who really wants to put poisons on their horse?) I will now only buy the Pyranha. Everything else has been a disappointment and a waste of hard earned money.