While trying to get my hextech chest today, i played a few games with my friends, 1 game i noticed was with lux getting an s+ (the lux being one of the friends I was playing with) at the end of the game i got no chest and was surprised, This happen last week with the same friend. ( i did check my champs and i had not gotten a chest with lux yet this session) i will start taking screen shots for next time should this keep happening.
When a friend in your premade group gets and S grade and gives you a chest, the chest is counted towards the champion that you are playing, not the one your friend is playing. This means that if you are playing a champion that you already have earned a chest on, you will not get a chest for this situation.
So while you may not have gotten a chest with Lux yet, you likely already had a chest for the champion that you were playing that game, and thus did not get another one.
I think if they get the score on a champ you also got it on then you won't get it. Idk if this is your case tho
