When microstepping, each microstep has 1/n the torque. So in 1/16 mode, each microstep is 1/16 the full step torque. Microstep torque is cumulative, so you do get full torque at full step positions (half torque at half step positions. So, it sounds like you aren't getting full torque out of the motor, or there's enough friction in the system that more torque is required?
What's the current rating of the motor? What current is the drive supplying? And what voltage are you running at?