When you are in filter mode some Building Blocks will appear faded, depending on your filter. It can happen that no existing Building Blocks are faded, only the new ones you make. If you are not filtering Building Blocks it is advised to turn the filter mode off.
Open the TAGS filters tab in the lower right hand corner of the screen. Once open, click DESELECT ALL to cancel all active filters. Repeat the same steps for the PARTICIPANTS tab, which can be found at the lower right hand corner of the screen.
Read more about filters here: Using Filters