Is it one cell out of the whole question or bucket where other options are showing up fine? My guess then is that cell is a zero and causing an error with running stats testing.
If it's a whole question or bucket or on an embedded data field then something about that variable is incompatible with tallying in a cross tab.
Qtip: Imported data and embedded data are compatible with crosstabs, but must be added to survey data before you create your first crosstab. Crosstabs are compatible with the Text, Text Set, Number Set, Number, and Filter Only formats of embedded data.

thanks so much. I'll look at the qtips that you have suggested.
it's weird because I'm running a whole series that are exactly the same set of stubs with only the banner changing each time (for age range, race, class, income). All the banners are bucketed responses to a single question - and all of them are working apart from the one set.
thanks again for your time.

Agree very odd. It may just be a glitch and rebuilding the one banner set that is giving you a problem could also do the trick. If you still can't get it to work I would reach out directly to Qualtrics Support staff. It may be a problem they can answer (I also didn't find documentation on the support pages so I gave you some guesses of what is wrong). If they can't answer then it may be something they bring up to their engineers.