SingularPlural
I had been walkingwe had been walking
you had been walkingyou had been walking
he/she/it had been walkingthey had been walking

SingularPlural
I had been sleepingwe had been sleeping
you had been sleepingyou had been sleeping
he/she/it had been sleepingthey had been sleeping

SingularPlural
There is no past perfect progressive for the "to be" verb. "Had been being" is expressed simply as "had been": "We had been being successful before, but we somehow lost our knack."