I'm having the same issue, looks like 1 of my 250mm strips is partially dud and so is one of my 450mm dud.
Corsair wanted me to post the entire kit back to them with plug etc and everything to examine for any potential warranty replacement even though its just the 2 dud strips, even more annoying is it's not possible just to by a single replacement for each strip and the pairs of them start at £26 and £40 respectively, honestly wish I had never bought them....
If you are thinking about buying them for your monitor... don't...