Upload not working correctly

I have been using Smart FTP for quite a while, but just last week it started going wrong.

after I have uploaded a couple of files, it then stops working.

If I drag a file to the server, the progress bar goes white and stops. Then eventually the file will appear on the server, but will be size 0.

After it does this once it won't work again until I close and restart SmartFTP.

It's become unusable, what can I check to get this sorted. Could it be firewall issues on the router?

Many thanks