Is the shared stream a stream that has been created by you, or did you accept an invitation? You can only add photos to streams that you created.
If you cannot add to the stream, you may have exceeded a limit, see:
If your stream alredy has 1000 photos, you will need to delete some photos.
You are welcome
As long as photo Streams are only one-way roads, create your own stream to send photos back. The advantage is, you will have control over who gets invited to view your photos. If you would add to someone elses Photo Stream, someone might be getting access to your photos that you did not expect to see them.