How detect thap PID working for the bed? (no replies)

Hello there!
My nozzle have PID activated of course. When it reach the right temp then the LED for it on the mainboard start flicker and the temp is perfect, stable.

My heated bed also had PID activated now. The temp reaches but after a while printing 6 min. something like that it starts to go down and never reach the correct temp again, even if the bed LED is lightning. I don't have the LED flicker as it is for the nozzle.

I have tuned PID with comand M303 E-1 C10 S120

In the printer display you can see the PID parameters for the nozzle but not for the bed. How to activate this?

I have a nice 12V even with load.

What can the problem be?

How can I debug this?

Thanks
Ola A.