Grammar - Past Perfect Continuous 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?

http://grammar.ccc.commnet.edu/grammar/tenses/past_perfect_progressive.htm