The hardware event logs retained by the motherboard's Baseboard Management Controller (BMC) are stored in a limited capacity flash memory. When this memory has been filled, you will usually find that SEL events are not stored. For example, if you save a SEL log and find that is has a few thousand entries, but the newest entry was some time ago, then it is likely that the SEL log is full.
Clearing the SEL log in the motherboard's memory will allow new events to be stored.
To clear the SEL log, use the SELVIEW utility, either from the Windows Command line or Internal EFI Shell - (or, for the older S5000 based server boards, use the SELVIEW utility from DOS instead of EFI).
Simply type in SELVIEW /clear and then follow the confirmation instructions to clear the log.