I don't see what the problem is. If you have the folder set to be scanned once, then that is what will happen. Picasa will scan it once, save the images in it and never scan it again.
What you want, as you yourself state in your question, is to scan always. That way, every time Picasa is opened, it will scan the folder for changes and import any new images it finds.
http://support.google.com/picasa/answer/11511?hl=en :
Select one of the following monitoring options on the right:
- 'Scan Always': Picasa views all compatible image types in these folders, and it automatically adds new photos. Picasa actively scans this folder for changes.
- 'Scan Once': Picasa accesses all compatible image types in these folders only once. It doesn't detect new photos added to folders with this setting.
- 'Remove from Picasa': Picasa doesn't look in folders that are marked with this status.