I don't know that specific board, but a common restriction for many NI data acq boards is to support only one hardware-timed Digital Input task at a time. (Also true for each of AI, AO, DO.)
I think you'll just have to capture all the data at some fixed rate, bearing in mind that some of the transitions may be captured nearly 1 sample later than they actually occurred.
There'd be a possibility to try change detection across all the DI in a single task, but there are some quirks there as well. If the changes are happening faster than the change detection timing can handle, they won't all initiate their own sample. More than one bit may have changed between consecutive samples. Other quirks are possible when bit changes are nearly but not quite truly simultaneous.
-Kevin P
CAUTION! New LabVIEW adopters -- it's too late for me, but you *can* save yourself. The new subscription policy for LabVIEW puts NI's hand in your wallet for the rest of your working life. Are you sure you're *that* dedicated to LabVIEW? (Summary of my reasons in this post, part of a voluminous thread of mostly complaints starting here).