Hello and welcome.
Most digital video content is protected by DRM (digital rights management), including DVD/Blu-ray discs, and all STB (set-top box) devices from cable/satellite providers, especially on digital outputs - HDMI/DVI/TOSLINK optical audio, etc. Recording from their outputs is in violation your "contract" with their service, and technically also violates copyright of the media itself. This is one reason that the digital age brought built-in DVR functionality, which replaced the old lower quality VCRs...to allow subscribers to record shows to watch at a later time, not to build a personal library. In addition, you have probably noticed that almost all TV shows are available for sale now on disc or via streaming services, which may have shifted licensing to be more restrictive with regards to personal recording.
My experience and understanding of any exceptions for educational use are more geared towards printed material, and showing/broadcasting copyrighted A/V material in class on projectors, video displays, etc. I am not aware of any legal way to manually copy/duplicate copyrighted digital video material for personal or educational use.
All that said, it may have worked for you previously, but it sounds like DISH Network and/or other device manufacturers may have implemented their copyright protection onto the analog outputs as well. If so, there isn't any legal way to go around the protection, as that would constitute breaking their licensing rules and possibly legal laws as well. Doing so would be reminiscent of folks that illegally hacked their cable box with third-party chips back in the day to get the "pay channels" for free.
Of course, I am not an expert by any means, nor do I have any experience with the workflow you are using. But, I personally am a supporter of DRM and all copyright laws, as many of my friends make their living in the music/movie/writing/photography/art industries.
There may be some information on the DISH support website regarding any change they may have implemented.
Sorry I don't have a better answer for you, but I sincerely thank you for all of your work in the educational field. It is appreciated!
C
P.S. My comments are not meant to condemn nor condone, just for the safety of all.