I agree, I'm a Red Seal plumber and have my gas B license. There are some DIY people out there that have a good idea of what they are doing but when it comes to things that can potentially kill you like gas venting you have to call a professional ticketed technician. It may cost $300+ to fix the issue, but it's cheaper than a funeral.

If it looks like the flame is coming back towards you when trying to light the pilot then there is a backdraft which is dangerous since if the pilot does light and the backdraft continues carbon monoxide would get pushed in to your home. Could be a clogged vent or improper vent but I would call a gas technician or plumber since it could potentially be deadly if not addressed.