When I read the lastest readme for DAQmx 9.2.3 release, I noticed there is one known error "Samples Not Acquired on NI 660x or NI 6624 Devices. When using pulse width, semi-period or two-edge separation buffered measurements with the NI 660x and NI 6624 on counters 0 and 1, some samples may not be acquired under certain circumstances". Actually when I was using USB-6251 to measure pulse period using counter 0 or counter 1, similar problem occured too. At that time I used buffered counter sampling triggered by digital rising edge and occasionally the available counter samples remains zero or stopped at some uncompleted sample number. I thought it might be the trigger problem. Is it possible the USB6251 counter has the same problem as stated in the readme?
