Why does this 1-counter frequency vi output zero occasionally

[Jeff in Pleasanton]

I've searched the forums and learned that the 1-counter-low-frequency vi from the digital frequency measurement library fits my needs (roughly 2500 Hz to 25000 Hz signal freq range, DAQpad-6016 device).  I thought all I needed to add to the example code was a while loop with a delay, and a gauge.

But the vi outputs zero fairly often, and I don't see why.  I don't think the input signal is dropping out, so it must be another one of those newbie mistakes I keep making! 

I've attached my vi.  Thanks for any insight!

Jeff