Me, my dad, my sister, and two friends of mine have all experienced this. We all have iPhones and Macs setup to receive iMessages. We experience this with other iPhone people. It's very random, like less than 1% of iMessages. My friend/dad/sister will call and say "hey why didn't you return my text?" I check and I didn't get it. I then go to my computer, open Messages, wait for it to sync, and the missing iMessage appears there. It never shows up on my phone. We've all done hard resets, turned iMessages off and back on, and no change. Mine is a 6+, setup as new. Two are regular 6s. The other are both 5s I believe. This is either an Apple server side iMessages bug, where it is only delivering to a computer and not an iPhone, or an iPhone bug where it doesn't get some messages sometimes. I thought about turning off text forwarding, and signing all of my Macs out of iMessages. That way if it's an issue of it being unable to deliver to multiple destinations, this may solve the issue. however, fi it's an iPhone bug, then I'll never get these messages since iMessages won't be enabled on my computer.