Apparently people aren't in my friend list?

Occasionally when I'm in the middle of a match and my friends message me, I try message back only to get a message saying that said summoner is not in my friends list. I check my friend list after match to see if they're still in me friend list. Yep they still are. It doesn't happen all the time but it is annoying. If anyone knows what's going on or how to fix it could they please post something? I would like to know what I can do about this.
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