could be a dozen different issues.
- dirty gas burners, incorrect gas pressure to the burners, iginiter dying or spark in the wrong place or weak etc etc
best to get a Pro to check it out. if you leave it longer then it may start to have delayed ignition/explosion on startup which is dangerous. taking the door off is altering the airflow over/to the burners enough to get it to ignite but that sounds dangerous to me and I have seen LOTS of burned up wiring from delayed ignition in my many years.
- dirty gas burners, incorrect gas pressure to the burners, iginiter dying or spark in the wrong place or weak etc etc
best to get a Pro to check it out. if you leave it longer then it may start to have delayed ignition/explosion on startup which is dangerous. taking the door off is altering the airflow over/to the burners enough to get it to ignite but that sounds dangerous to me and I have seen LOTS of burned up wiring from delayed ignition in my many years.