Generally if one fails the lot fails! That's always been the problem with series-wired plugs, and that's why many have changed to the newer type which are connected in parallel, because if one fails, the others keep going!
I remember that there were some very expensive plugs around a zillion years ago, which switched the failed plug circuit internally, which allowed the remaining plugs to continue working. It also lit up a warning light on the dash to let you know that you were running with one down ... Only it didn't tell you which one, and due to the switching it was easier to replace the lot rather than try and find the one that didn't work!!
@Series71 You use either a full set of Series or or a full set of Parallel glow plugs, and cannot change anything as they are designed differently! Trying to do so usually means a nasty, expensive burning smell !!