I uploaded a lot/ a Sim to the gallery but even after a few hours it does not show up there
The most common cause for this is that the gallery filters out lots and households that contain custom content. The gallery has a mechanism that checks if your upload as custom content and then marks your upload as such.
Sometimes stuff without custom content is categorized as containing custom content. For example are pictures of custom content are also classified as custom content.
Photos and paintings that were painted by reference count as custom content and are flagged because of that.
There are other reasons like issues with the gallery but those are mostly temporary.
Check the box Advanced – Include Custom Content. This will make the gallery show all uploads.
Unfortunately this setting is not saved so you need to re-set it each time you visit the gallery.