Try this - each time you download a new demo thru your proxy from the store or pc ensure you use a "free demo" link that you have not used before. Also, if in 1.70 make sure the downloads are performed in the background. You need to be quick when downloading small activation files (100k). When downloading activation files you can use the same link that you used to d/l other files of the same title.
Some of these points are mentioned in other areas on the forums, but thought having it in 1 post may help others. Worked for me.