[Forkster]

Here's the breakdown of the work to troubleshoot my bike - I've owned it a year now but haven't ridden it much. Changed the oil last year and only have put 500km on it.

Tuesday week before - bike wouldn't start and battery was dead.
Thursday - replaced battery. Bike still wouldn't start - just turns over.
Sunday - took apart bike, checked voltages, checked plugs. Bike suddenly started - put back together. Tested one hour later - all good.
Monday - bike wouldn't start again. Pointing to probable faulty HES?
Tuesday - Followed a HES guide on ADVRIDER.COM on the no-start. With stand down, in neutral, turn on bike, hear pump prime, press start. No start - just turns over. With in gear, clutch in, stand up, turn on bike, hear pump prime, press start. No start - just turns over. Take off plugs, put in plug tester, press start, no spark at plugs.
Thursday - Built my HES tester. Tested my HES - cold testing HES indicates it's functioning (sigh). Decided to look in the airbox. FML. It's a K&N and it looks like it hasn't been cleaned in a while. Cleaned the air filter, recharged it.

Apart from the HES (and a really dirty/clogged air filter), is there anything else that would cause the plugs to stop working suddenly?