Did you ever contact VTI and find out exactly what spring they sent you?
A standard Uberti main spring should set off all brands of primers with no problem. As we have said before, standard Uberti springs are stronger than need be to overcome internal friction. If you have two springs, and if they are standard springs, and they do not reliably fire primers, then something else is wrong.
If you remove the spring, and work the hammer, do you feel friction or resistance to the hammer moving? There should be light resistance as the bolt pops over the hammer cam, but other than that, there should be very little resistance to the hammer falling.
What about the firing pin? Did you try replacing it?