My iCal events intermittently disappear since upgrading to Capitan

I've seen many people having similar issues (not necessarily with the upgrade) but I haven't seen many answers. I add an event in my iCal calendar (on my Mac). It is synched with my Google calendar which also synchs to my iPhone and iPad. I check back a few days later and sometimes the event has disappeared. Does anyone know how to fix this? I rely on my calendar for everything. I could start using just the Google calendar instead, but I'd love to be able to use iCal as well. Thanks.