Welcome to the forum. Yep you're right there is a bit of a bug on the box/stick if you're trying to watch some episodes via the Latest section of the NOW TV app for the first day or so they appear.
So for example going to Entertainment > Channels > Fox > The Walking Dead > Latest > Ep 13 when it first appeared didn't work, although it does work now.
You can get it to work if you ignore the Latest section and look for it in the Season instead eg if you go via Entertainment > Channels > Fox > The Walking Dead > Season 8 > Ep 13 instead it should work. Or you could look for shows you watch regularly in My TV > Continue Watching instead.
The same goes for any other shows you are finding don't work via the Latest section.
It has been reported to NOW TV so hopefully it will be fixed soon.
@Lydrc You're right Ex on the Beach doesn't currently work via the Latest section but it is working for me if you go via Entertainment > MTV > Ex on the Beach > Season 8 > Ep1/2.
Other people have commented in the past that the Catch Up section often seems to be behind and not all catch up episodes seem to end up in there anyway so I always ignore that section and find shows I want via browsing, searching or my watchlist / continue watching list. However there are some episodes from yesterday in there eg The Flash. What's the first few episodes showing in catch up for you?