Before Fuelly can calculate MPG for individual fuel-ups, it needs a full tank to work from. Fuelly calculates an individual fuel-up MPG by using the difference from full. Your first fuel-up was marked as a
partial, so Fuelly can't calculate an MPG for that fuel-up or the next one. The fuel-up you just added was full, so on your next full fuel-up you'll get an MPG calculation. Here's more info:
Why do I have to fill the tank all the way up every time I buy fuel?
And here are some other scenarios where you might not get a calculated MPG for a particular fuel-up:
Why doesn't my fuel-up have a calculated MPG (L/100km)?