Arch-gun suddenly does low damage against PT?

[willthiswork]

Yesterday, my dad and I took down the Profit-Taker Orb a few times. Between my rank 22 Velocitus and his stream of curse words while trying to get the arch-gun deployer to work, we didn't have too much trouble. Today, I decided to do some PT missions on public and all of a sudden my now rank 30 Velocitus doesn't do much of anything. I crit one of the PT Orb's legs for all of 32 damage. Even against the regular corpus in Orb Vallis, my Velocitus did almost nothing. I took it to a random mission on Europa to see if it was broken there, but it was just as devastating as it was yesterday. It just seems to be broken in Orb Vallis. Any idea why? The only things I've changed since yesterday was adding Magma Chamber and Resolute Focus.