When you say heat-up continuously, did you measure the temperature? MOSFETs can run safely when very hot to the touch. How hot is the case, or if you can calculate it, the junction?
Some possible causes of the overheating of the low side switches:
1) Not enough drive voltage on the low-side switches.
2) Slow turn-off or turn-on time (possibly too much gate capacitance) in the low-side switch.
3) Conduction of the high- and low-side switches overlap (though would expect the high-side to overheat too).