Recently active topics in Wuala tagged with files remain local

I have tried to upload these files to a group several times (and then also tired uploading them to my (private) files. Whenever I tried, the encryption process runs well and then they appear in the folder where I wanted to upload them to, but:
- there's no upload activity
- when I look at their properties (in wuala), their state remains "local"

This only happens with certain files and I still have space and can upload other files just like before.

Could the reason be that those files are on wuala already (could be possible, they were once publicly available as free downloads)?
Or is this some sort of bug?