Uno and CNC Shield 3.51

I am lost. I am running a Arduino Uno Rev3 and CNC shield 3.51 to run my CNC router.
I originally had a makita router as the spindle and I have recently switched to a 2.2kw spindle with VFD.
Everything works correctly even the PWM to control the spindle. However the PWM doesn't reach 5v only about 4.4 to 4.5v so I don't get the full 24000 rpm out of the spindle.
I have pulled the shield off the UNO and measured the Enable pin on the UNO and have 5v actually 5.05v when S is Maxed.
Could someone point me in the right direction to achieve the 5v from the shield.

Thanks in advance.

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