I have a couple of eggs that I’ve picked up from some Pokestops (I’m lucky enough to live above one).
I walk a lot. I get to work through a 2 mile walk / train journey every day, so I should be picking up loads of steps for the eggs I have. However, because I’m normally always running late… for everything, I have to rush everywhere, so I walk at an above average pace.
Today I’ve walked around 5 miles, but the game has registered 100 metres. Is this because I walk too fast, or because of something else?
It’s worth pointing out that I have the app open when walking… mostly because I have to catch them all. (This made me miss a train this morning… luckily I was early :D)
Based upon my time playing Ingress (created by the same company and also using a lot of the same infrastructure it seems), you can go at least 15 mph (~24km/h) and it SHOULD still track you. (This is a little less than the 30 kph someone in the comments mentioned).
Note that 15 mph is a 4 minute mile (or 2.5 minutes per kilometer). This is a little slower than the record running speed, and still over the average biking speed.
It also doesn’t tend to lock you until you’ve made multiple jumps at faster speeds, and it only does so to stop spoofers.
Therefore I doubt it is your walking speed.
If your game isn’t registering your distance, I would assume it is either GPS troubles, the game in general, or server issues. The majority of this is handled by Niantic and will likely improve as the game progresses. Unfortunately, that means there is probably little you can do other than contact Niantic to let them know.