When I overflow buffer, it works only if buffer has 20 bytes length. More than it, doesn't do its job

First, this is NOT a black hacking attempt. I am trying to learn about buffer overflow. I have doing my own TCP vulnerable server. It just receives data in a buffer, and copy it to another more small.

I have tried with a 20 length sized buffer. I send this shellcode:

mov eax, 0xDEADBEEF
mov ebx, 0xDEADBEEF
mov ecx, 0xDEADBEEF

I have sent:

 15 bytes length of shellcode
5 trash bytes to reach buffer limit
4 bytes to overwrite EBP
4 bytes to overwrite EIP (ret addr)

But doing this with a more big buffer, suppose 60 bytes, when I put needed trash to reach buffer, the app overflows, but it doesn't run anything. And the EIP points to a unknown place.

I am looking for tutorials, but they don't tell how address this issue.

Thanks for your help.