The most common cause of this is that you have not synchronized the Zotero on your computer with the Zotero server. You need synchronise Zotero with the Zotero server regardless of which file storage solution (Zotero, WebDAV, or Dropbox) you use.
If ZotPad does not synchronize with Zotero, the first thing to check is if Zotero on your computer is properly synchronized with the Zotero server. You can do this by logging into Zotero.org and then clicking on "My Library" link on the top. Check if the online library is up to date.
If the online library is not up to date, check these instructions: http://www.zotero.org/support/kb/changes_not_syncing
If there do not seem to be any problem in the online library, but ZotPad still does not synchronise properly, you should use the in-app support system in ZotPad to start a new support ticket.