Hi Sybille,
Sorry about your recent rejections. Perhaps I can shed some light on this rejection reason.
In general, the "that is not what we are looking for" rejection goes beyond simply subject matter. It's usually reserved for a combination of factors, technical and artistic. In fact, the subject matter might be fine if it is shot in the right way.
I have not seen your rejected images, so I'm writing this in the general sense of what I've seen overall.
Many of the images that get this rejection usually have less than ideal lighting (direct lighting, bad shadows, etc.) and distracting backgrounds. Most of them are images that were shot outdoors.
For instance, someone submitted images of a unique portuguese sea vessel. They were rejected for the aforementioned reason. Why?
Well, they weren't really images of this unique vessel. They were snapshots of a boat that happenend to be this unique vessel.
The photographer failed to capture the essence of the boat. They were just snapshots. Direct sunlight casting bad shadows, flat composition with distracting background and foreground (parts of the dock and bridge in bad places), etc.
What makes this boat unique? If it is its shape and size, shoot it so that you show this the best way possible. Is it a quick boat? Is it a powerful boat? Use lighting and composition to create a shot that tells us this. What role do the dock and bridge play? Are they necessary? If not, don't include them. If they are necessary, incorporate them in a way that doesn't conflict with the main subject.
By doing this successfully you accomplish many things. Not only have you created an aesthetically pleasing image of the vessel but you created a conceptual image that symbolizes quickness, strength, and beauty. The image has thus become much more universal and saleable.
So when you get this rejection, say to yourself:
Did I truly capture what makes this subject appealing and saleable? Does the lighting and composition bring this out? Are there any distracting elements in the photo or something that doesn't add to the photo? If the elements have a purpose to the photo, do they complement the main subject or compete with it?
I'm not a reviewer. This is just my two cents based on what I see being rejected with this reason.
Thanks!
-Steve