If your getting issues with uploading via FTP, your server may have disc quotas enabled which will restrict the amount of FTP user can upload.
First, log into your Plesk control panel and then click on Domains from the main menu
Select the domain name.
Click on Websites and Domains tab
Now click on FTP access
Click on FTP user that you wish to check/increase the FTP upload limit.
Enter the new ftp upload limit and click OK to complete change. see note below
Please note: If you see the following then disc quoting is not enabled and this is not the cause of the upload issue.