Target Display Mode working but not shortcut Cmd-F2

Hi,

I have a iMac 27 2010 model with macOS Sierra and target display mode is working fine with my MBPr 2013. It's also working with my pc which i have connected using a mini DisplayPort to DisplayPort cable. This works automatically, when i power on the the pc the iMac turns immediately to target display mode.

The only problem is that i cannot switch on/off the target display mode with the shortcut Cmd-F2 from the iMac. I also tried Fn-Cmd-F2 but nothing happens. In order to switch back, i have to either power down the pc or disconnect the DisplayPort cable.

Currently i'm using a matias usb 2.0 keyboard on the mac, since my original mac keyboard died. I also installed Karabiner keymapper to check what is pressed when i press Cmd-F2 or Fn-Cmd-F2 and it seems that the correct keys are pressed.

Don't know if it's relevant with this topic, but from Yosemite (i think) Shortcut for sleep mode (Control-Cmd-Eject) stopped working (a lot o people have the same problem)

Any ideas would be much appreciated.

Thank you

Teo