General Discussion - Anyone else have trouble distinguishing between retroflex initials?

Z and Zh

S and Sh

C and Ch

I can clearly hear the difference on radio and tv.

I clearly hear the difference when my prof says them.

But for some reason, I have run into many Chinese people who pronounce 四 and 是 exactly the same. Their Z and Zh both sound like Z, and same goes for C and Ch.

Is it just my ears not working right? Or is it physically impossible for them to pronounce Zh, Ch, and Sh and instead choose to turn them into z, c, and s?

I find it so frustrating speaking with them because they say to them there is a difference between what they say, yet my ears hear exactly the same sound.