Check to see if you've inadvertently set a speed limit on the loco:
Menu/Control/Max Speed
Then, from page 36 of The DCS O Gauge Companion 2nd Edition:
• The screen will display the engine name and number, and the current maximum allowable speed for the engine
• Turning the thumbwheel up or down adjusts the maximum allowable speed
• Press the thumbwheel to lock in the maximum speed and return to the CONTROL menu
This and a whole lot more is all in MTH’s “The DCS O Gauge Companion 2nd Edition", available for purchase as an eBook or a printed book at MTH's web store!