Sealing Gas heater brevis underground duct

Hi everyone

Question about Brevis gas heating undergound - was recently installed by previous owners (<1 year old) but live in a relatively large house - 4 bdr - and only 1 room and the toilet heat up well - all other areas lounge and other 3 bdrs dont heat up. Overall there are about 8 ducts around the house- 2 get good warm air flow the others are very mininmum (but there is some flow only).

Just wondering what are people's thoughts

- i cant see any obvious leak (shone a light at night through the ducts) a fiber light i pushed through the tunnel

-I was planning to consider this option: The house is arrange in two large areas- so going under the house and closing the whole duct to the 2nd half of the house physically- so the heating is only to the lounge and 1 bedroom and toilet
1. Just wondering how I would be able to physically close off the area by going under the floor- do i just cut the duct at the area and just place a piece of plywood over and seal it - maybe use some foam as well as per https://ecomasterstore.com.au/products/duct-sealing-kit

2. Was thinking of this method- https://www.youtube.com/watch?v=aCCWAmESK4w

3.My research shows it can cause pressure effects and affect the mechanics- but really the duct heating is quite bad so i'm happy to take the risk

Thanks!