Firebird - I have not used the tool, but also had to deal with loose hup-caps.
Are you bending down all the tabs, or just a few? I found on mine that just bending four at noon, 3, 6, and 9 o'clock works the best. Another tab combination is using just three tabs equally spaced.
If the bending system of three or four doesn't work, I then wrap a thick rubber band around the tabs; then as you bend the tabs down they compress the rubber band.
I hope one of these will help.
PS. You didn't state if you have 21" or 19" wheels. Mine are 21".