I am having the same problem now. Any solution proven to be working?
I was having the same issue, and in my case it was caused by the browser cache. So the problem was that the browser was showing the cached image instead of requesting the updated one.
I solved it by adding an expression to my WebScreen with the following value:
"<meta http-equiv='cache-control' content='no-cache' />"
This tells your browser not to cache any content on that page. Don't forget to set the expression's "Escape Content" property to "No", so that it gets rendered as html.