The Model 975 only supports 3D in side by side. Ensure your source device is outputting this.

The Model 976 does not have any video processing and will simply pass-thru any content, coming from a compliant source, that is within its bandwidth. As HDCP 2.0 and HDCP 2.2 are technically not backwards compatible, your 3D source may need to be connected to HDMI input 5, which is HDMI/HDCP 1.4, rather than input 1-4.