Your circuit is wrong. I have numbered the transistors Q1 for upper left, Q2 lower left, Q3 upper right.
When a' and b are High, transistor Q2 is on and Q3 and Q4 are going on/off alternatively as the PWM signal goes H/L.
When Q4 is on, the motor is acting as a generator and so current is flowing through Q4 and the lower left diode. Since there is not much resistance in this loop, a relatively high current will flow and so I suspect that Q4 will rise out of saturation and thus be dissipating significant energy.
You need to ensure that Q4 is off when Q3 is off. Likewise for Q1 & Q2.
Also, the 1N914 diodes are not suitable. I would use power diodes such as the 1N4004.
Len