Procedure needed for initial fuel-up for new vehicle.
Recently I acquired a new car. When I took delivery, the dealer the tank was filled by the dealer. Knowing that Fuelly ignores the first reported fuel-up since it does not have a starting odometer figure, I reported the dealer's fuel-up as a full tank and the odometer reading at the time of purchase. Later when it was time for MY first fuel-up I reported it as usual but Fuelly still ignored the starting odometer reading and did not calculate L/100k or MPG.
If you start with a full tank there should be some way to tell the system the starting odometer reading so every fill gets included in the stats.
Every fill is included in the stats. If you don't see an MPG rating on a specific tank, that doesn't mean that fill isn't taken into account. When Fuelly calculates average MPG for a vehicle, it uses total fuel volume filled and total distance driven.
The MPG on an individual fuel-up is just an "as you go" marker. If the previous tank was full, Fuelly can calculate the difference from full and the distance traveled and give you a sense of how it went. This method of calculating fuel economy isn't precise by any definition of the word. It's simply a way to see how you're doing.
I understand the motivation to be completist and track every last drop of fuel that has ever been inside a fuel tank. But we're not doing anything here that requires that level of precision.
You might switch to tripometer tracking instead. That would give you an MPG calculation every time without fail. That way you could be in charge of knowing when your tank is full. With odometer tracking, Fuelly needs one "on the books" before it can calculate an individual MPG for a tank because the system has no idea how far you've traveled.
Another option: if you know for sure that the dealer filled the tank all the way at a specific odometer reading, add that as a fuel-up without a price (which you won't know). Then Fuelly can give you an individual MPG on your first fuel-up.
Yes, I did an initial entry on acquisition October 25 )or possibly a day later) showing the initial odometer reading and a full tank. I can only speculate that your system rejected the entry for some reason but it did give any error message at the time.