You have checked that the new matrix fittings are the same as the original? And that the original is easy to fit?
Perhaps the gaskets are distorted?
Or the new ones aren't the right shape, and you would be better off using the old ones with some sealant - the cheap matrix I fitted to the Omega required that.
Checking today. Both old and new matrices fit OK, and the pipes fit in them easily without gaskets.
One original gasket was destroyed on dismantling, the other fits well. The new gaskets are not identical to the originals. I tried gluing them into the new matrix, then I could not get the pipes in at all.
I visited my local Vx dealer, only to learn that the gaskets are not available alone, they only come in a £250 kit.
I tried reconnecting the pipes and matrix, the old washer fitted well, but the new one not so well. could not get the flanges close to each other.
Looks like some delicate surgery on a new washer might solve things, or just make it leak. Food for thought.