CD nozzle does not choke
 

Hi, I am designing a supersonic intake at Mach 2.5. I have two questions

where can I provide the back pressure for this case?

when I decrease the Mach number from 2.5 to 2.3 the normal shock appears and moves toward the convergent section but when I increase the Mach number back to 2,5 the normal shock does not appear in the throat. Why is that and how do fix it? The inlet is a mixed compression inlet.