Vent pipe as mentioned above sometimes gets spiders or other little critters making a nest and block the hole like mud cap. Be careful blowing out the hole with compressed air as all the debris (if any) go in the tank and then clog pick up tube)
When I put in a new tank I added a drain and a top nipple for a roll over valve. Allows tank to vent but if rolled over shuts off so no leaks and fire.
(Tank may had had the vent there also new- it’s been a while)
