Are you able to look in the Chrome development console and see if there are any errors reported there? You can write
Puck.debug=3to show all the data being transferred which may give some hints.
If you can some code running every so often that is writing errors (or non-JSON formatted messages) to the console, that could throw things off. You could connect to the Web IDE and leave the watch for a few minutes and see if anything gets printed.
End of last week I did make some big changes that should help with this - I guess it's possible you were using a version before then.
I am running v12, and apps seem to pause in the middle of uploading. If I reload the page I have a small chance of getting some apps through, but the vast majority of the time it pauses without any progress several minutes later. Any ideas?