My I7 6700K no longer has a bosst speed

My CPU I7-6700K no longer boost's to 4,2GHZ

I have checked the bios for issues also re flashed the bios and checked some settings and yet still will not boost. Reinstalled windows 10 64bit

I have ran prime95 to see if it will boost using that and still sticks to 4,0GHZ

I'm using a asrock pro 4 z170 I have also tried my other rigs motherboard being the Maximus viii formula z170 And still the same issue.

I have also had system locks with this issue well putting two and two together

Any help would be great

4.2GHz is the max turbo for a single core. When multiple cores are stressed the max they'll do is 4.0GHz. So if you have one application that use 1 core, than yeah you will see that core turbo boost to 4.2 GHz.