Dear @arsalanbuilder, @tahasoomro & all,
All those who are getting low mielage issue on cng,it is recommended to clean/clear the gas cylinder
When you will clean the cng cylinder, u will be surprised on seeing quantity of liquid oil inside your cylinder.
when oil will be totally drained out then u will see that more gas quantity will be filling in your cylinder (although the pressure bar says 200 on your meter but it i showing combined pressure of cng + liquid oil + other impurities & containment particles/gas etc.
so get your cylinder serviced once in a year atleast to get free from oil
(That oil comes from the CNG filling stations who have weak compressor)
Oil coming is the sign of Weak compressor