The centre pin on the 510 connection on genuine Vivi tanks is adjustable, gently, with a small screwdriver. If it is not slightly proud of the surrounding metal (1 mm is fine) you may not be getting proper contact. These can move if you screw the tank on too tightly.
On older LTs (the ones without the sloping shoulders) airflow can be a problem with some tanks (not just Vivi Novas) and a plinth with adapter may help.