In the Star Trek: The Next Generation episode "Family", actor Theodore Bikel appeared as Worf's foster father, and actress Georgia Brown appeared as Worf's foster mother. Later in the series, in the episode "New Ground", Brown reprised her role, yet Bikel did not appear.
Why did Bikel not appear again in this role? Was he unavailable (and for what)? Not interested in the role? Asked for too much money (he had been the president of Actors' Equity)? Became two feet tall at four o'clock? (I'm looking for an out-of-universe answer regarding the actor, not the in-universe reason given for his character.)
He died in 2015, so that's not the reason.