Hi Tony,
Let's check a few things. First off, make sure you have the latest versions installed which is 1.16.9 for your Desktop and 5.1 on your iPad.
Next, let's check that both sides are signed in to the same account. In Logbook Pro Desktop look at the far right side of the menu at the top of the screen, it should show you signed in with your yahoo account. Now in the iPad version 5.1, go to My Account and see that it is signed in to the same account.
If you have data on your device (iPad) go to Settings/Synchronization and tap the red button at the bottom to mark all as unsynced. Otherwise, add a few test entries to verify sync works.
Return back to the Home Screen and tap the red Sync button (red means you have data awaiting sync)
Verify your iPad is syncing by logging in to
http://logbookpro.com/sync and see if your app data is present.
Now from your desktop click Cloud Sync / Sync Now to sync that information into your desktop logbook.
You should see your data.
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.