Heater output is really low, burners not lit

Hi, all. After 7 years of solid operation, we noticed our heater not putting out the heat it used to. The attached spa used to take a few minutes to heat, and now takes about half an hour. We also noticed a little odor coming from the heater. I finally got around to looking at the heater today, took the service panel off and got down and looked at the burners. I noticed that only 3-4 of the 8 are lit. There's a gas pipe that feeds all eight. The first one on the pipe is not lit, then the next 3 are, then the one after that kind of flicks on and off.

I blew compressed air in the burners, and scrubbed the fittings coming off the gas pipe with a toothbrush and WD40, to no avail (one burner further down the line fired, but not energetically, and I can't say for certain whether is had fired before I did this or not.)

What's the most likely cause here? Thanks in advance.