Why is panacur not working?

Hi, I recently aquired a beautiful saluki puppy, 6 months old, who's owner had to give her up. They assured me she was wormed up to date, and I believe them, she was jabbed and well cared for and they obviously loved her. I did notice she was small and thin for her age, I have had saluki's all my life so this caught my attention.

Anyway, the first night I had her she vomited, small amount of fluid writhing with worms (no exageration) I immediatly wormed her with max dose Panacur, then a week later, as per instructions wormed her over 3 days - in this time, she vomited only once, vomit contained just a few worms.

Today, three days since her last treatment she vomited again, I swear half her body weight is worms, there were maybe 100 of the little cuddlebugs writhing around in her puke. Sigh.

I'm 99% sure they are roundworms, I've collected up a baggie full of the little darlings to be positively identified. They are all different sizes from tiny to 4-5 inches long.

I called the vet, have to take her tomorrow for 'special medication, heart check etc, my question is, why did the Panacure have no effect, I gave Maximum dose for her weight as per instructions and stuck to the regime?

Has anyone else had this problem? did your dog recover ok ?

many thanks for reading, I know it's long and gross and I much appreciate your time and answers