1) Do the attachment files exist locally in the source library? Check the attachment presence indicator in the items list. If you created the items on another computer and have file syncing set to download on-demand, the files might not yet exist if you haven't yet opened the file on this computer, and Zotero doesn't currently download the file automatically before copying the attachment item to another library (though I believe we have an open ticket for this).
2) If you dragged the items previously, either before the attachment item existed or when the attachment file wasn't present locally, you'll need to delete the target item and empty the trash before dragging again. (This is an longstanding annoyance that we need to fix.)
This happened again. I have a collection in My Library and am trying to copy of the items into a Group Library. It copies over the items but doesn't bring any of the attached PDFs.
Upgrade Storage
1) Do the attachment files exist locally in the source library? Check the attachment presence indicator in the items list. If you created the items on another computer and have file syncing set to download on-demand, the files might not yet exist if you haven't yet opened the file on this computer, and Zotero doesn't currently download the file automatically before copying the attachment item to another library (though I believe we have an open ticket for this).
2) If you dragged the items previously, either before the attachment item existed or when the attachment file wasn't present locally, you'll need to delete the target item and empty the trash before dragging again. (This is an longstanding annoyance that we need to fix.)