Addressing the last issue first, the asset you are importing may have the same name as an asset that has already been imported into the project assets. Even if you delete the asset first and then reimport it, it’s usually better to give the new asset a different name.
As for the missing assets, this is usually due to the asset not being in the same place on your computer as it was when you first imported it. There are two ways to ensure this doesn’t happen. The easiest fix is to embed the assets into the project before saving. However, this can make you project file quite large. Another option will be to save your project file in its own folder and add an assets folder into the same project folder. You can then copy all your project assets into the assets folder. When you next open the project, it’s easy to reconnect your assets from your assets folder. From then on, you shouldn’t see any missing asset warning’s. Just keep your assets folder inside your project folder along with the Blocs project file itself.