I'm trying to calculate the Mahalanobis Distance (to check for outliers) in SPSS for a scale of 25 items. When I use the Regression function and save the Mahalanobis Distance values, everything seems to run correctly and MOST of my cases will have an MD value of some sort. About 10% of my cases do not (the cells are simply blank) and while some I can maybe explain away in terms of missing data for specific cases, there remains a substantial chunk of cases where I can't figure out why the cells for the MD values are blank.
I've tried looking at each of the individual cases and there doesn't seem to be anything strange about how the data was entered for the specific scale items.
Can someone please explain to me what is happening and if there are any potential fixes to this?