As I commented above, it is not RW that is the issue here, it is your ISP. I can and do upload 6 streams at once when I am running my VPN. When I am not on my VPN connection and just using a standard ISP connection I only get one active connection and the same issue you do for the other 5 connections.
I would suggest you experiment with different VPN’s to see for yourself. You might also try different FTP software. You will experience the same result. To get the issue resolved will take you speaking to your ISP to have this issue resolved from their end or just bypass them and use a VPN.
Not sure who your ISP is. I use Spectrum Cable. Ever since TimeWarner was bought by Spectrum I have experienced this issue.
Don’t get me wrong, there are a lot of issues that RW needs to work on but this is not one of them.