If reindex fails or is interrupted for some reason, there might be an index left which is not used but prevents further index updates, e.g.:
Looks like the index has more than one identifier. You should delete all but the one of them currently active. Here is the list: frwiki_content_1494688722,frwiki_content_1495187567
We should make a maintenance script (or option in one of the existing scripts) which would detect which of these indexes is currently active and delete extra indexes. It is possible now to do it manually, but it is prone to error and deleting wrong one by mistake may cause search downtime.