get your throttle body serviced ... too much carbon build-up is not allowing air/cng fuel to get into the intake.
also get your gas cylinder checked for oil ... if there is oil in your cng system then also gives trouble.
if the above options don't work then get your tappets adjusted.
if you get a check engine light for the O2 sensor being lean ... then just get the cng mixture adjusted. not a big issue.