For some time I've been unable to add some users to some of my private Twitter lists. The lists are nowhere near Twitter's limits for list numbers and the problem applies to all the platforms I've tried: the Twitter Android app, Hootsuite Android App, Tweetdeck and the Twitter website itself. On some platforms (the apps) I get a message saying "Cannot add to list". In Tweetdeck there is no message at all. It doesn't make any difference whether or not I'm following the users in question and they are all public accounts. I'd be very grateful for any help with this - this problem is making it very difficult for me to get the most out of Twitter at the moment. My Twitter ID is @andrewzcooper