vibration sensor gives no value after sometime

Hello

I am using NI PCI-6024E card for acquiring voltage using vibration sensor(KS80D).First when I start to  acquire values from Vibration sensor, it gives me data fbut after some reading it gives constant value from sensor.Then When i Reboot my computer and start to acquring the voltage it acquire again but same thing happen after some time it gives me constant value.I have attached two pictures for more understanding.I do not know what is the problem.It it problem with overloading or somthing else with sensor?Please give me solution about this problem