Common causes for your scenario of not seeing prior data:
1) A backup was restored over top of a later data file thereby causing your data to reset back to the date of the last backup
2) A different file or windows login was used such that the data was synced into a different file than what you are using now
3) The filter bar in the Logbook Pro Desktop spreadsheet area is active and filtering what you are seeing. On the sub-toolbar just above the spreadsheet log about 7 buttons from the right is a funnel button, make sure it is NOT pressed in.
4) The dates on the device flights were not entered correctly such as using a prior date so the data ended up higher in your spreadsheet log area than expected.
Open Logbook Pro and go to Cloud Sync / Settings and sign out if you're signed in, if not, then sign in. Make sure you are signed in to the same account as you use with the app and login to NC Software with, then click Cloud Sync / Sync Now to sync in data from Cloud Sync.