Ok. Try changing spark plugs, and put an injector cleaner in your fuel tank.
Look, personally, I think the Liana was never meant to be driven on CNG. This is like driving a Belta or Vitz on CNG. PakSuzuki made a BIG mistake with an extraordinary car. We need to change our mindset on this matter, its a psychological barrier that CNG will ALWAYS be cheaper than petrol.
I always treat my Liana like a JDM, and I think thats a good approach. Between you and me, replace air filter, plugs, oil etc. etc., and drive on petrol only. Reset the ECU so it can relearn your patterns. You will find near CNG equivilent fuel economy.
Let me know if it works, learning all year round.