Target Disk mode "restarting holding T"

[Olegphoto]

How do I restart in target disc mode with MAcbook pro , firewired to another mac, restarting holding the "T". It is not working, I have to go to system preferences > startup Disk > and do Target Disk mode.

Before I could just hold the "T " on restart and it would do it. How can I make that function work again?
THX

 

[Loa]

Hello,

When exactly, and for how long do you hold the T?

Loa

 

[666sheep]

OP, reset PRAM. It should bring things back to normal.

 

[Olegphoto]

I'm holding it for a while , I know how it works I did many times, it just doesn't happen.
I can't reset the Pram, holding all command/ option/p/ r
Doesn't do it neither.