Dimmer switch not turning on light until at 50%?
Many dimmers have an adjustment for the minimum level. You don't mention what type of dimmer you have but you can consult the instructions on how to adjust this. As @dslake mentions in his answer, adjustments might be via a knob or screw, but if it's digital, it could also be a button sequence.

Needing to perform this is more common with electronic dimmer switches since they need to work for many different types of bulbs (incandescent, CFL, LED).