TCP Window Full when using SCP


So, we are troubleshooting a wan issue. I can FTP back and forth between two servers over the internet. It seems slow, but it works. When I try to SCP that same file it starts out good, but slowly dies and disconnects. I’ve run wireshark on it and the beginning looks good but then I start getting “TCP Window Full” packets. Once those start appearing I lose my connection. I’m not completely sure why the window is full. Anyone have a clue? Also, is the source IP of the “TCP Window Full” packet, the one that is full or is it the destination? Thanks in advance!