that WAS not a big issue but dangerous ! the result was that the CNG pump was faulty and thats y that was happening , i ran the car and utilized all the cng and then i checked the valve pin of the CNG cylinder , it was shinning and no traces of any carbon / zinc or anyother thing, i pushed that pin with something and it was very fine working , then i went to CNG repairing shop and the guy said Sir the valve is all ok nothing wrong in it . and then i went to SHELL cng and get it filled with no fault at all .
so all that was the faulty cng pumping station .
jazak ALLAH