If you sign into your account in a web browser at https://lightroom.adobe.com/, do you see the affected photo? Usually, photos with sync problems are listed in a special folder called "Sync Errors." Do you see anything like this upon signing into the Lightroom Web app? Is there a specific error or help message displayed? It could be something like "open Lightroom on iPad to finish syncing."
If you do, then try the following advice:
1. Does restarting the iPad app help sync finish? (I mean force-quit, not just simply returning to the home screen and then resuming the app). See this for steps: https://support.apple.com/en-us/HT201330
2. In LR iPad app, does applying a small edit or a caption to the photo let syncing finish?
3. If none of the steps above helped, then please try re-importing the image. First, please be sure you still have the original image backed up safely somewhere outside of Lightroom. Then delete it and empty the trash inside Lightroom. Then re-import the image. Unfortunately, this process will cause any previous edits or other changes you already made to be lost.