Two common issues with using NDI as a source are NDI version, and Magewell Pro Convert device naming. 

NDI Version Compatibility

Most NDI encoding devices allow the user to select what version of NDI to use when creating their stream. Users should be careful to select a version with which their version of Hippotizer is compatible.

NDI Version Support

NDI HX is supported from version 4.5.5+

NDI V5 is supported from version 4.7.1+

NDI V5.5 is supported from version 4.8.5+

NDI V6.2 is supported from version 4.9.1+
(From https://www.manula.com/manuals/green-hippo/hippotizer-v4/4.9.1/en/topic/ndi-receiver-on-layer

Magewell Device Naming

There is a known issue with Magewell devices to do with their default naming scheme, which uses a special character that causes Hippo to see the source, but not receive video. You can solve this issue by renaming the Magewell device to something simple, like "Magewell 1", in the Magewell settings UI.