I see alot of people say they aren't getting all texts and I also had this issue but not for a long time! I quickly figured it out.
if you’re using a phone administrator such as 'Yahoo' or 'True caller' make sure you turn it off, as it marks several messages as junks! otherwise, open your messaging app, (for iphones) tap the back arrow to see the menu and go to the junk folder! All your unseen messages should the there.