No able to achieve 1.8 Degree in 4-wire 1.8 Degree stepper motor

Hello all,

I am using a stepper motor with following specifications :

Steper motor with 2mm pulley attached to shaft compatible with GT2 timming belt
Step Angle : 1.8° Degree
Configuration: 4 wire Unipolar stepper motor
Holding Torque: 2.1 Kgcm at 1A per winding
Phase current : 1A
Resistance/phase: 2.9 ohm
Rated Voltage : 3.3V
Inductance/Phase : 4.7 mH
Length : 44mm
Shaft Length : 15 mm

While running motor using spartan 6 FPGA, I am not able to achieve 1.8 Degree and minimum degree I am able to achieve is 17 Degree.
Kindly suggest procedure by which I can achieve 1.8 Degree.