Yeah, on those attempts where it doesn't appear - that'll be because it didn't refresh during the fight, and so isn't written in the log for WoL to pick it out. Same happens with any auras, such as Soul Link that aren't refreshed or applied while the log is writing, or during the combat period WoL is looking at.
For example here There is no Soul Link uptime, but I always use it. here you can see Soul Link was used; it's simply that I applied it between pulls that it didn't register in the logs during combat. The same thing happens with Demonic Pact. Where it does appear with very low uptime may be due to ranging, or it's source switching from Ebon Imps and Infernals.