BackEMF for BLDC is not correct

Hi all,
Im working on BLDC sensorless algorithm using zero cross detection but i have a weird problem, the back emf appears when duty cycle is 100% is

and with duty cycle less than 100 is

while ideally when phase goes from low to high or vice versa, its in slope form, but it is not the case with mine.
Im very new to motor control so pardon my silly mistakes ut can somebody plz explain this why the BEMF for me is not like this?

is it bcoz of the saturation in the stator windings or something else, and if so how can i make it like the last image.

Thanks for replies

Ashutosh