Vespa — Self Start doesn't work, Light Dims and Horn sound fades when we accelerate

We have bought a new Vespa ZX on 12st October 2020. After our first service we have observed weird issues that Self Start doesn't work most of the time and even we have to use kicker 2-3 times to start the vehicle. 1 month ago it stopped all of a sudden in the mid of the road and didn't start at all. We took it to the Dealer and they said fault in the battery and they fixed it. But now the also the self start doesn't work. Moreover when we accelerate the lights are dimming out and the Horn sound fades. There is some serious problem which either vespa dealer couldn't find it or fix it. No other bike gave us any trouble even after years, but this is brand new, costly, reputed and it gives us nightmare.
Sad part is I am not able to raise a complaint in vespa site.

VID : TN 88 V 8454
Dealer : VKG Motors, Karur, Tamilnadu