Sometimes it is hard to hit the right spot on that enemy mech. The hitboxes of different mechs are different and sometimes not that obvious.
Hopefully this will help you finding that seet spot you are lookning for on the enemy trash can.
IS mechs are found here
Clan mechs are found here
Yellow = cockpit
Green = Arms
Red = Legs
Orange = front Side Torso
Blue = back Side Torso
Purple = Center Torso