Cuttman,
As always I would suggest you follow the usage instructions that comes with the product.
As for one product not working, perhaps you have set your expectations to high.
No product can eradicate all flies. They can only help control the infestation as there are to many variables to be considered which the product can not overcome.
Such as:
Location of rubs
Herd size
Animals per acre
Lack of pasture rotation & pasture size
Application procedure ( pore on vs. rub vs spray on)
Geographic location and climate conditions
Application regularity needed
In some years a rub may be all that is needed, in other years or time of year a regular spray program or a combination of both may be needed depending on your other management programs.
If you Google cattle, fly control, acceptable level I am sure you can gain a good understanding of what an acceptable level is.
Also you might want to Google life cycle of flies.
Hope this helps