Well, I use Chomp and do not have this issue. Maybe there's a conflict in your settings somewhere that is preventing messages from being pushed, and the phone is waiting until a certain event to push them all at once. I have no idea what that certain event is, other than being on cellular data and not WiFi. If I woke up in your shoes, I'd do a Marshmallow app & data backup and a factory reset to see if that clears it up.