With CPUs it's a lottery, sometimes it's not even the motherboard or the cooling. Obviously the main thing is the cooling but a mix of proper cooling, motherboard and a good cpu will get you good overclocks.
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Sounds like you have reached your CPUs Max overclock. I had an i7 7700k that couldn't go over 4.8ghz even though it had more thermal headroom. You can go into your bios and try to adjust your overclock manually, in my experience this has always gained me better clock speeds and lower temps. But as it's been said before it's the silicon lottery.