If the add-on receives streams from only a single source (M4U, 123Movies, etc.), it would be possible that source is down and in this case you have to wait for an update. On the other hand, when the add-on is multi-source (Exodus, SALTs), the reason would be one of the following: you have not installed the add-on directly from the repo or you have picked an uncommon movie/TV show that has no streams available.
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