"Knowing" you had a lucid dream but you can't remember it isn't uncommon for people with lesser recall. Keep a dream journal; stay in bed for a few minutes after waking up to try remembering what you dreamt about, then get up and write it down immediately. If you wake up in the middle of the night, make sure there's a sheet of paper and pen at your bedside, so you can just reach over and write everything down without actually having to get up.