CAL's scenario does not say whether Nevill checked on June, Daniel, Nicholas & Sheila before following Bamber downstairs.
It is doubtful he did as it would have given Bamber the time to re load & return.
It would have been a pretty instant decision to follow rather than wait until he returned upstairs.
Knowing himself & June had definately been shot, Nevill must have felt the priority was stopping a re load.