There could be many reasons for cPanel not receiving emails:
Is the attachment an executable (.exe)?
Executable files are blocked from being sent through emails, this is to prevent the spread of viruses. They will be rejected by both the sending and receiving servers.
On rare occasions email clients may reject attachments if they have settings to not get emails with attachments off server.
Logged in on Multiple Devices
When logged in on multiple devices, be weary as it only takes one device to mark an email as spam which will cause that email to be moved on all devices.
Other (See Links Below)