Make sure there is no debris in the pilot light. It looks rusty, and it is possible a flake of rust fell into the orifice.

Note that the the screw that says "pilot adj" can be turned increase the gas flow going to the pilot light. But, I doubt that the screw being too "closed" is your problem. Be careful if you turn the screw to increase the pilot light gas: your pilot light is not lighting or not saying lit can create a fire hazard, and turning up the gas flow can make it worse.

As for back pressure, do you have anything like a cloths dryer or exhaust fan running, and has your house has been better sealed up recently (like adding lots of spray foam insulation/caulking/weather stripping)? That can cause air to get pulled down the chimney into the house.