Undervolting alone won't.. or, shouldn't keep your CPU from boosting. Have a look if you didn't disabled anything by mistake.
How did you undervolted? A fixed value? Then yes, you maybe hitting a wall.
Either do it with negative offset or curve optimizer.
As always, have a look at this video, starting at 7:18
https://youtu.be/yVFjvizChrw?si=mqOOD2p9I5-YiGnF
The Englishman