Ok, too much speculation and no answers-

You have to hold the derailleur cage OUTwards and THEN tighten the cable on. You'll never get satisfactory cable tension otherwise.

The other option is to use the Low adjustment screw to hold the derailleur OUTwards the middle chainring (so you don't have to hold it there) and then tighten it.

And when I say outwards, I mean away from the bike, as in moving towards the middle and big chainring.