This question is in reference to Atlassian Documentation: Using JIRA Software for Resource or Portfolio Management: why am I seeing a gap of multiple sprints for a resource in Portfolio, even though there is no other story for the resource and no dependencies?
I just spent quite a bit of time trying to figure this out and what I found was my release start date didn't match my sprint start date on the sprints that were getting skipped. I found that while I had started all my releases on Mondays and ended all releases on Fridays, the sprints I created for each team ended on Fridays but started on Saturdays. The sprints that were getting skipped where the sprints at the start or end of a release where this mismatch was apparent. When set all my releases to end on Fridays and start the next on Saturday just like my sprints, this problem resolved itself.