I had the same problem last week. My gas stove could not get a rolling boil on 3.5 gallons. I am hesitant to cover the pot because of boil-overs. It can take hours of scrubbing to get that stuff off the stove.
If you can try and position the pot over 2 burners. That will help to get more heat.
I recently ordered an outdoor propane burner for my next batch. It is 185,000BTU's, which is opposed to 7,000 BTU's on my stove burner.