Had the same problem–not sure why the * doesn’t get it, but I found that chaining the * populate query with a specific field populate query works.
In my case, I have a business logo inside a component. The “business_information” component had several other fields that were showing, but the logo was omitted. To get it, I made this call:
/api/venues?populate=*&populate=business_information.logo
It’s a bit weird, but * apparently doesn’t mean all all 