It varies from label to label, and even then it's not guaranteed that everything will get a repress. Data Discs, for example, are generally good at repressing sold out releases, but there's been a few titles that haven't been repressed yet, such as Super Hang-On and OutRun.
There can be a myriad of reasons why something will not be repressed, but it's rarely something we can know about for sure up front. High production costs, licensing restrictions, poor initial sales, etc. Usually the best we can do is try to give an educated guess as to what will receive a repress.
Are there any titles in particular you're wondering about?