Make sure you are dimensioning on the same plane on both parts. Remember when you mirror a part "Top" plane on the original part will become "Bottom" plane on the mirrored part, or "Front" plane will become "Back" plane, depending on the face you chose to create the mirrored part.
Create a projection view from the view you are dimensioning, then project the new view again in the same direction, and try redimensioning.
Good lock.