Starts on CNG but troubles on petrol

More likely to be fuel pump if it is the cause.

Try turning the ignition ON and OFF 2 or 3 times before cranking the engune to see if it helps.

If the vehicle works fine on CNG then there is probably a fuel, Petrol, system problem.
If it is hard to start you should get the fuel pressure checked as this is a common hard starting problem. You may also have issues with the fuel injector size as they genrally need to be larger for CNG as compared to Petrol. Unless the vehicle has been set up to run both, you may want to check into this. But perform the fuel pump test and fuel pressure test first. To test the fuel pump and relay, you may need an assistant. The pump primes the system for about 2 sceonds when the key is in the, "ON: position. It may be hard to hear the , "whirring " sound of the pump, so have your assistant get near the fuel tank as close to the fuel pump as possible and sometimes a large screwdriver will aid in hearing if you place the tip to the tank and the handle to your ear. It acts like a stethoscope and it will make internal noises much easier to hear. If the fuel pump is having issues priming it will be intermitant as the vehicle does run eventually. So, performa this test several times. Another issue you may have when going from CNG to Petrol is that Petrol turns to a varnish like substance after just a few weeks. If you have just a little in the tank it will turn even faster and can clog fuel filters and fuel injectors. You might ry running a high quality fuel sytem cleaner for one or two bottles.
I am giving you a diagnostic link that goes over hard to start issues in more detail. Some of them may not apply as the performance when running CNG is fine. Vacuum leaks and air tract and exhaust tract blockages are not as high on the probability scale as the other issues they go over.
You might start off by getting the trouble codes pulled to see if anything comes up as it is the best diagnostic tool you have and easiest to performa. Then move onto fuel pump and pressure tests. Read the diagnostic walk through first as it will cover these in detailk and it may give you some more hints as to what the problem is.
Here is the link;

https://www.2carpros.com/articles/engine-cranks-excessively-before-starting