It's possible that iMessage might not be deregistered in some of those cases -- the users can double-check that via a site that Apple provides:What I have found out is the Android user has gone from Android to IPhone and then back to Android it really stops some text messages. It is really bad if the text is part of a group text. It is an issue that Apple is never going to fix, as it is another reason many people “have to” stay on the Apple train. It would be great if Apple made sure your text was delivered no mater what platform you or the recipient were on.