My 2km egg is stuck at 1.37km and when I walk it stays at 1.37km. I have already tried restarting the game but it still doesn't update. How can I fix this?
There are two important factors at play here - how and when Pokemon Go calculates distance.
PoGo isn't a fitness tracker - jogging on a treadmill won't do. You have to go outside, and then move slower than 10 miles/hour (18kph). Every few minutes (based on testing, this is around 4 minutes) the game queries the difference between your current position and your position 4 minutes ago, and credits you that distance in a straight line.
So to hatch your egg, walk or bike slowly in a straight line.