The PAST PERFECT PROGRESSIVE TENSE

The PAST PERFECT PROGRESSIVE TENSE indicates a continuous action that was completed at some point in the past. This tense is formed with the modal "HAD" plus "BEEN," plus the present participle of the verb (with an -ing ending): "I had been working in the garden all morning. George had been painting his house for weeks, but he finally gave up."
Generally, progressive forms occur only with what are called dynamic verbs and not with stative verbs. If you wish to review that concept now, click HERE.

Singular	Plural
I had been walking	we had been walking
you had been walking	you had been walking
he/she/it had been walking	they had been walking

Singular	Plural
I had been sleeping	we had been sleeping
you had been sleeping	you had been sleeping
he/she/it had been sleeping	they had been sleeping

Singular	Plural
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."

Hemingway had been losing his self-confidence for years before the publication of Old Man and the Sea.
Had they been cheating on the exams before the school put monitors in the classroom?