Hi gunter, usually it's only planar ends on things that will get capped automatically. With a non-planar end shape like you've got there it will usually require you to custom build the cap surface yourself rather than it getting automatically filled in.

But if your capping shape is straight in one direction (like it looks like is the case here, since it has a single silhouette in the front view), it's usually better to form such things by initially making an extended shape and then doing a boolean difference with a 2D profile curve to cut off the end, leaving the imprint of the 2D curve's extrusion as the cap.

I'll see if I can make an example for you.

- Michael