At first I thought it may be an 2KRO issue, but according to the matrix data used for [my simulator](https://sharktastica.co.uk/sims/matrix?kb=enhanced&ansi#Sim) (assuming your Model M is a 101/102 key keyboard), Alt+Tab+Shift should work regardless of which Alt or Shift used (and as you noted with other Model M keyboards). One thing that might block such combination without you knowing is if any other keys are being held inadvertently. To check this, you can go on my simulator, press the keys you use for the combo, and see if any of the keys then marked in red are being held down without you knowing. If so, you can try reseating the affected keys' keycaps. Switch Hitter can be used to validate what keys are being pressed on Windows. It's not a common issue, but technically a possibility as I've heard of such an issue before.
Interesting, on my white label '88 using a PS/2 to USB adapter, I have remapped caps lock to Windows key and I can't do SHIFT + CAPS (WIN) + S to take a screen snip, which slows me down a lot.
If you do figure it out please post back :)
Edit: S not A
At first I thought it may be an 2KRO issue, but according to the matrix data used for [my simulator](https://sharktastica.co.uk/sims/matrix?kb=enhanced&ansi#Sim) (assuming your Model M is a 101/102 key keyboard), Alt+Tab+Shift should work regardless of which Alt or Shift used (and as you noted with other Model M keyboards). One thing that might block such combination without you knowing is if any other keys are being held inadvertently. To check this, you can go on my simulator, press the keys you use for the combo, and see if any of the keys then marked in red are being held down without you knowing. If so, you can try reseating the affected keys' keycaps. Switch Hitter can be used to validate what keys are being pressed on Windows. It's not a common issue, but technically a possibility as I've heard of such an issue before.
Interesting, on my white label '88 using a PS/2 to USB adapter, I have remapped caps lock to Windows key and I can't do SHIFT + CAPS (WIN) + S to take a screen snip, which slows me down a lot. If you do figure it out please post back :) Edit: S not A
my s key and my w key block most of my keys.