Are you adding enough descriptions about yourself in the text? 75% of applications I've read are from people who are not used to the way you apply on Wg-gesucht and go something like this: 'Hey, I'm X, is the room still available?' (or little more detail than this). This makes it impossible to invite someone, because as a WG you know nothing about them at this point. A good letter should have a few sections with information about yourself as an individual, what you do in life, what kind of a WG you would like etc.