It's pretty common to occasionally pedal mtbs without being clipped in. Rubber on the sole around the cleat makes a huge diff. I have some two-hole CF soled road shoes that I tried with SPD mtb cleats. It was nearly impossible to pedal without being clipped in.

One trick I sometimes employ when starting out on a hill or in other pressing situations with only one foot clipped in is to push down with that foot, lift it up turning the crank backward 1/2 rotation, and stroke again. That's a lot quicker and allows applying more force than bringing that foot around the rear of the crank.