No power on cold start after putting cat back on

Had my decat removed on my ktec system yesterday and the standard cat put back on, test drove it straight after and it was fine, the flat spot wasn't as noticeable, was all good but came to drive it earlier and i live on a hill, started the car, set off, soon as i put it in 2nd it died, had to drop it to 1st and dip the clutch to get it moving with some revs. My thinking is that its leaking around the join at the cat?? obvs gonna take it back when i get chance, does my thinking seem logical?

Craig