Not sure what you're asking - they may be incredibly slow typists? Or they hesitate? Or they re-read what they're about to send? Or maybe they're not giving you 100% undivided attention?
Or are you saying that you're watching your conversation and a message pops up w/o seeing "XXX is typing"?
How can you be sure that it is only showing that the other person is typing sometimes? Are you receiving messages without seeing that little box saying they are typing? Maybe the messages they are sending are very short, and that is why you are not seeing it? To the best of my knowledge, it does work all the time. At least on my phone!
