Some streaming Apps/services will not permit use of external displays. In such circumstances, while audio may be heard, you'll not see a picture.
Other potential causes are HDCP (High-bandwidth Digital Content Protection), this being a Digital Rights Management (DRM) anti-piracy technology. Where implemented, all devices in HDMI chain must be HDCP-compliant. If one or more devices (such as the HDMI Adapter*, TV/Monitor or an intermediate HDMI device do not support HDCP, you'll not see a picture.
(*) Both the Apple Lightning to Digital AV Adapter and USB-C Digital AV Multiport Adapter do support HDCP.