Some individual subscribed channels limit the number of simultaneous streams they will support on one account, spread over all devices (phones, tablets, web browsers, various streaming devices). My Netflix subscription allows 2 at once for instance, Hulu 2, Prime 3, etc. You may have one that will only support 1 stream at a time -- my Hulu used to be limited to 1 but I just read it now is 4..
Make sure you don't have an active stream of the same channel on some other device, perhaps going unwatched.
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