why sometimes the ipo actuator can't be activated

i made some actions by ipo and tried using two methods:
1.use only one actuator and set the frameStart and frameEnd by python and use controller.activate() to activate it
2.one ipo actuator for one action and use a property to activate one of the actuators. use a python script to change the property.
both use a delay sensor to make sure not changing actions too often.
i use two property to moniter the ipo actuator and find that the actual activation times is less than the action changing times. the second method is better than the first one but it still has the same problem.
any ideas ?

Attachments

1_ipoctrl.blend (130 KB)2_ipoctrl.blend (135 KB)