It seems as though you can only reply to PM's via the Waze iOS App the first time you look at them (via the pop-up which appears on the left-hand side of the screen). Often times, I must do a bit of research prior to replying to the PM. However, I am then unable to reply to the message by pulling up the message in my inbox. It would be very, very nice to be able to reply via a "Reply" button in the inbox (otherwise I have lost my opportunity to reply to the PM).