Lost some signals when using NI6210 and labview

[bigben542]

I am using NI6210 and labview to sample the output voltage and current. The output voltage going though the voltage sensor board  is connected to the NI6210. When I use labview to observe the voltage waveform,some sampling signals are missing. The PWM frequency of the original voltage signal is 10KHz. My sampling frequency range is from 20KHz to 60KHz.  

I am pretty confused with the missing signals I detected, do you have some suggestions? Thank you very much. 

I used the standard high frequency (20KHz) squarewave to test the setup and it works well. However, once I sample the output voltage as you can see below, there are some signals missing. Is that because of the inaccurate sensed voltage I get from the voltage sensor board I made by myself? 

The figure above shows original voltage（top）and sensed voltage(bottom).

The figure above shows the voltage signal in labview.

[James.M]

X-posted over here