Sometimes it's not just the permissions, it's the Owner. We had similar issues. Did *you* create the assets and uploads folder, or did you let SilverStripe create it? If you created them, then try removing them completely, then doing a dev/build and try going into the CMS and uploading some files. This should make SS automatically create whatever folder it detects is missing, and then the webservice will be the owner of that folder instead of you.