I have several custom buildings for Cities Skylines but in the "Content Manager", in the "Assets" tab it doe not shows all the assets that I owned. Is there a problem with my game or the assets itself ?
Depends. Are these assets you created yourself or that you downloaded from steam? If the latter, I would say that steam has been plagued with congestion due to the game's nodding popularity it may just take a bit longer after you subscribe. Also, when subscribing, make sure that you have the checkmark on the assets' picture AFTER you back out of its description page on steam. I had an issue where I was hitting subscribe on the description page but wasn't registering immediately when I backed out.