Ever since updating to Lollipop, Google Hangouts has been giving me problems. I never receive messages from others when they are sent: they all arrive very late and I'm bundles, and I have to exit Hangouts and reopen it to get messages to show up. This is very frustrating, because Hangouts works just fine on my tablet, laptop, and Chromebook.
From what i gather by browsing around, and experience, the notification delays are due to WiFi. On the VZ network, i get everything fine, on wifi (work or home) I won't get them, or they will be very late.