So, I've exported all my PSTs to a folder (E:\PSTs) and I've installed the PST Capture Tool. I've successfully transfered a whole load of PSTs to Exchange Online, however, I'm getting the following error on a lot of the mailboxes:

Import error: The specified object was not found in the store.
Now, what is unusual, is that on some of the mailboxes, if I try it and let it fail and then retry the import a day or so later, it goes through without any failure. However, I still have a lot of mailboxes where it won't import at all. Any ideas how I can get resolve the error?