PWM'ed DC motor not running when loaded
If you look into the design of the H bridge driver you may be surprised at how it behaves under load conditions. Your PWM ratio is 80/255 = 0.313 and you'd have thought the driver, powered from 5V would produce: -

Vout = 5V x 0.313 = 1.57 volts. But this won't happen. Under light loads I estimate the driver is capable of swinging up to about 4.3V and down to about 1V - this means your assumed target of 5V is in fact more like 3.3V making your real output dc voltage (due to PWM) more like 1.04 volts.

It gets worse under higher load currents. The data sheet specifies that the output can swing up to 3.6 volts and down to 1.2 volts when supplying 1 A. This means, at your specified PWM ratio, the output voltage will be: -

\$\dfrac{80}{255}\times (3.6 - 1.2) V = 0.753 V\$

So, without changing the PWM ratio, at light loads your motor is fed with 1.04 volts and as mechanical load increases this will drop to about 0.75 volts at 1A: -



It's not a good driver for your application really and it's worse if you use worst case (rather than typical) values.