Unable to set FS breakpoint in VSE 2019.4

Hey I am trying to debug an OpenGL FS from Visual Studio 2017, in Windows 10 on an MSI GS65 laptop (GTX1070), using VSE 2019.4, driver 441.20.
I can open the shader source, and insert a breakpoint (or plenty of breakpoints) but execution never stops there. I know execution must go through those lines as they are part of the output colour calc (or indeed, where the FS assigns the output colour) and I can see the output colours when I run the app…
After selecting a pixel in resources, then going to the shader and inserting a breakpoint is there something else I need to do?
Thanks!

Al