"Sh" can be a tricky sound to teach! I have a sweet girl with Down syndrome who has a lot of nasal emission when producing this sound. In fact, she is unable to produce anything sounding like "sh" when she pinches her nose closed. It almost comes out more like /k/. This shows me that 100% of the air comes out of her nose when she normally produces the sound. We are going to start working on producing the sound in isolation without nasal emission.
Have you worked on nasal emission before? Let me know if you have any advice!