Heartland Metro 3805 4 burner gas

[Fritz]

one burner won't light with the top cap on, no matter the valve postion.  Swapped caps, no change.

cleaned the burner rings and area, n/c.

Lights fine with top cap off.

[citizenX]

Not familiar with that brand but can you observe the electrode when it is sparking compared to the burners that are lighting properly? Is the spark jumping away from the gas flow when the cap is on? Cracked ceramic on the ignitor maybe? Bent or dirty electrode? What about any clogged tiny holes or ports that feed gas towards the electrode? Just throwing a few things out there common to gas cooktops in general. 

[Fritz]

spark seems to be the same as the others.  No cracks in ceramic. 

The electrode is bent a bit, but I think that's the design.  They're all <5º off the vertical.  Cleaned the electrode with my fingernail.  Not much came out.  Can it be scrubbed with a toothbrush & 70º alcohol?

The ports are all clear.  Both the top ring and the larger bottom ring.

It's odd because perhaps 1% of the time it fires, NP.  It's a back burner and not used as much as the front 2.

Heartland was a Canadian company that got absorbed and deleted by Aga.

At the time, it was the only 30" with front apron controls, as we had safety concerns with cooktops with the controls on top.