I have a older HP notebook that's doing same thing. It can reach base frequency but not turbo boost. I verified this through a third party CPU information app. Everything is enabled in UEFI bios and I finally installed Ubuntu just to see if Intel turbo boost worked and it did. The CPU in the notebook is an Ivy Bridge 3427u duel core 4 thread 17Watt CPU. Its specs say it supports Turbo boost 2.0. It did work in earlier Windows 10 releases such as 1809 but either in 1909 or newer releases I began to notice the slow down.
Tried also to create a Performance power profile and set min/max CPU to 100% to no avail. Because my Bios makes no option to turn on or off turbo boost I assume its always on. After getting nowhere with WIndows 10 I decided to stick with Ubuntu because my CPU is a legacy CPU now so Intel won't support it.