When does a job get skipped and does it ever re-run?

Hi, I've had some complaints lately about jobs not running. A couple of questions... 1) How can I validate if a specific job was skipped? 2) Do skipped jobs ever re-run? 3) How can I monitor all my skipped jobs? 4) For summary indexing, how do I recover that data? Thanks a bunch! I will be looking at the doc for these questions, but, as you know, Splunk almost has too much doc!