My right Joy-Con will not connect as a separate controller?

My switch's right Joy-Con works perfectly fine when playing in handheld mode, but when I go to set it up so I can play a game locally with a friend the light pops on after a bit but it doesn't respond to me pressing L and R at the same time on the controller

Help a fella out
Update: I did get a new controller at one point and it could connect to the switch as a separate controller fine, but after a while it stopped working as well
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