Time delay probelm in data aquirsition of voltage measurement

Hi,
I am measuring voltage using NI DAQPad-6015. Without time delay loop, I can get the data. But what I want to do is to compare the data with that of following moment.
The delay time loop is used to aquire the next moment data.  With this loop, it just stay at DAQmx AI Voltage. What's the problem?
Attachment is my program. 
Attachments:
DriftTest.vi ‏79 KB

Got it. It works. Thanks a lot. 
 BTW: what are the differences among Wait(ms), Wait Until Next ms Multiple , Time Delay?
Alyssa

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