Emails not being transferred to Gmail properly

Hello, I am using Thunderbird to migrate the emails from one Gmail account to another. I had an issue with the size of the .mbox file due to the number of emails, so have split the .mbox file into several smaller siles. The file I am transferring has 811 emails in it, when moving this from the Local folder to the Gmail folder, all of the emails are moved across successfully. However, when I go to my Gmail account, only 516 emails are shown to be there. Has anyone come across this issue before and found a solution? Thanks in advance.