Desi Jr. left the series to concentrate full time on his movie career. In later seasons Lucie was exploring other career options (live theatre, music) and that's she wasn't in some of the episodes.
Only a life lived for others is worth living. Albert Einstein
A life isn't worth living unless it has impact on other lives. Jackie Robinson
Be always at war with your vices, at peace with your neighbors, and let each new year find you a better man. Benjamin Franklin