Mac screen capture with Intensity Shuttle

I'm trying to use an Intensity Shuttle for Thunderbolt to capture the screen from another Mac at 1080p.

The Mac I'm attempting to capture from is connected to the Intensity with a Mini DisplayPort to HDMI cable. I've verified that the display settings on the Mac being captured match the project settings in Blackmagic Media Express.

I'm only able to get a 1080p/1080i/720p signal when attempting to capture from an iMac or higher-end MacBook Pro. When trying to capture from various MacBook Air models or a lower-end MacBook Pro, I get a black screen at anything higher than a 480p output.

For what it's worth, I can capture a 1080p/1080i/720p signal just fine for every Mac I've tried when using an UltraStudio Mini Recorder. However, the setup I'm creating requires use of the HDMI output from the Intensity Shuttle.

Any ideas why the Intensity Shuttle would not be able to capture an HD signal from certain Macs that are otherwise capable of outputting such a signal?

Thanks.

Well the maximum 1080P you can do with the Shuttle is 1080p30 so you need to make sure the output from the Mac is set to that, any higher and the Shuttle won't see it

Well the maximum 1080P you can do with the Shuttle is 1080p30 so you need to make sure the output from the Mac is set to that, any higher and the Shuttle won't see it

Yes, I've tried that and several other settings. It works with some Macs, but not others that are otherwise capable of outputting such a signal.