If its a recent easy box, there is a good chance other people attacking it are breaking it.
There isn’t an easy way people can remotely answer this - so many things could be involved - such as firewalling, network connectivity, box issues, etc
You can troubleshoot it by running tcpdump on your attacker box to watch the traffic to see if anything happens (resets etc). Depending on the box you could run tcpdump there as well and see if is differrent.
You can try using curl, nc, scp or other tools to move the data - it might show up where a problem lies.
You can double check you have the ability to write data to the location you are downloading it to etc.
Millions of things to try, unfortunately.
However you can try other methods of sending the data. Try base64ing the file and then
echo "base64 stuff" | base64 -d > filename on the remote box. If the box has gcc you can copy the source files into a text document and try compiling it. Etc.