Not talking about your kneecaps(sadly I have actually been desperate enough to try that and fucking ouch), talking about vein behind your knee, the leg equivalent to your elbow crooks. Not painful at all, though it is a little tough to hit on your own, takes some contorting.

I've never heard of someone never able to hit their crook veins and/or finding that area painful, that's really strange/interesting(and a bummer for you). It's usually the easiest, most reliable spot for people and they usually last for years if you've got decent technique. Wonder what's going on there for you. Have you watched other people hit that spot? Has a nurse ever hit you there to take blood?

Yeah, I got a terrible abscess, probably the worst one I've ever had, in that same forearm/elbow spot you did, it's easy to slip out there cause the veins roll easy ime.

I know being overweight can make finding veins tougher. Have you looked at any vein maps(just google)? That might help you. Instead of going by what veins you see(though that helps too), it's generally better to go by feel. Veins feel bouncy/springy if you press on them lightly with your fingers.

Good luck man, be safe.