It can be frustrating, but the gallery can also be quite buggy at times.
I know you can save sims and builds locally via the gallery, but I sometimes share builds to guarantee that they'll be saved *somewhere*, even if something happens to my local save file.
So maybe the uploader just made a few changes to the house and reuploaded so they could keep it saved, and for some reason the gallery didn't credit you as being the original creator?
Just something to think about.
I totally feel your pain though. Especially when you bulldoze a lot where you uploaded someone else's creation, build on top of it, then have the build credited to someone else. Happened to me more than a few times.