Joycon isn't recognized as being "attached"

Having the same issue right now.

Replacing the contacts is one thing you might have to take into consideration. It depends on where the connection error comes from though. If you have some extra pair of non-faulty joycons flying around try them out, if the switch doesn’t recognize them either you’ll have to replace the contacts in the Switch’s rails. If otherwise, you’ll need to replace the joycon’s contacts.

You can try to reconnect the battery to the motherboard on the joy-con and if that does not work then make sure the ribbon cables for the rail on the joy-con are attached correctly to their ports on the motherboard.

Chiming in... (sorry to hijack)

I have been having problems with this for years. For me it was always the left joycon side. Bought some new joycons, and sometimes it helped for a while, but even the fairly new joycons started seeing issues after awhile. When playing handheld the Switch would somehow lose connection to the joycon(s) (I think really it's the left side), so I'm having to play with joycons disconnected, or play on the TV. I also got a pro controller which is my main controller nowadays. But still it would be great to be able to play in handheld mode again, or even be able to charge the joycons when connected to the Switch main unit.

I've tried cleaning things

• tried cleaning the connectors on the joycons, by using a white "plush" thing, squeezed down to be more flat to be able to fit into where the connectors are. I've used some hand sanitizer alcohol on the plush.
• tried cleaning the connectors on the left side of the main Switch unit with the same method as above; flattened plush drained in alcohol.

I'd say the cleaning only maybe helped like 5%.

I have three left joy cons.

• The worst of them, the blue one, just will never wake up again, it's dead, doesn't charge, the switch never recognizes it while trying to connect, even wiggling it around when connected to main switch unit.
• Another green one, is like at 30-40% of it's ability. The cleaning seems to help a bit, and it sometimes connects while attached to the Switch, but playing it in handheld mode often causes switch to lose track of it and cancels play and shows that controller connection screen/it suggests to play in disconnected mode etc. Being careful when attaching it, I can sometimes have it be detected and it will charge, so I can leave it like that for a while which means it will charge and I can at least play safely disconnected later.
• The newest, a red one, is maybe at 75% of the ability, it works pretty OK but the big problem is in handheld where again Switch will easily lose track of it and it again cancels play and shows that joycon connection screen etc etc. So in the end it's in fact impossible to play in handheld mode. Again it's fairly possible to have it charge and use it to play in disconnected mode later.

The gear is long out of warranty but is still very much in use by me and my kids.

I'd hate to have to pay tons of money to Nintendo for having them fix an issue which I think they are responsible for in the first place. Well who knows, things break, but it seems this is such a common issue.

Would it be cheaper to let some non-Nintendo shop fix this? Would it be safe or would I risk things by having them break things more? (I would probably trust Nintendo more to fix things properly)

Trying to fix it my self seems like an incorrect way to deal with the problem (The switch works well to play in disconnected mode, and I wouldn't want to risk damaging any other parts opening the Switch etc), but maybe in the end it could be worth a shot?