Next to 'New refueling' option, in 'Refueling' there is a section 'Unrefueled legs'.
Click on it to find out which fuel receipts have not been added yet. Click on to go to the 'Refueling' screen and add missing data. You can always add a new refueling for unrefueled legs by using 'new refueling' option and specifying airport, aircraft, date & quantity of fuel.