Question about signal extraction

Dear Chao-Gan, I was using DPARSFA to extract time courses from the AAL template. In the preprocessing, I used Head Motion Scrubbing Regressors and chose the default setting: Mean FD power > 0.5, regress out one TR before and two TR after. After the time course extraction, I encountered a problem, that in some participants, the extracted signals corresponding to some ROIs are NAN. They were not some particular ROIs. Different participants had different cases. I tested this after eliminated all the cerebellum ROIs assuming that the cerebellum may not be fully covered during the scan, but the issue existed in quite a lot of subjects. Is that normal? How can I fix it? Thank you very much! Sincerely, Xiaosong