Imported content disappeared

I recently noticed a lot of my imported content are gone, leaving a lot of my collections empty.

Were they purged, or do some content expiration rules exist?

Thanks.

We have had to purge some imported content of free members in the past when faced with storage issues. Free members being technically only entitled to 5 imported lessons. However, we have made changes to our storage abilities and have not had to do this for some time. This should not have affected any paying members.