Had been waiting, had just left

depuis + imparfait: had been doing

English had been waiting looks like a plus-que-parfait. French disagrees: if it was still going on at that past moment, it is imparfait, with depuis for how long.

• Marie attendait le bus depuis vingt minutes. - Marie had been waiting for the bus for twenty minutes.
• Il travaillait là depuis dix ans quand l'usine a fermé. - He had been working there for ten years when the factory closed.
• Nous habitions à Lyon depuis 2015. - We had been living in Lyon since 2015.

Two other ways to say it keep the imparfait too: Ça faisait dix ans qu'il travaillait là. and, more formal, Il y avait dix ans qu'il travaillait là. The depuis page covers the present-tense version.

venir de + imparfait: had just

Venir de + infinitive means have just done. In the imparfait it means had just done: the moment right before the next event lands.

• Je venais de sortir quand le téléphone a sonné. - I had just gone out when the phone rang.
• Elle venait de s'endormir quand les voisins ont crié. - She had just fallen asleep when the neighbours shouted.
• Nous venions de finir quand ils sont arrivés. - We had just finished when they arrived.

Venir carries the tense and the action stays an infinitive: elle venait de partir. The venir de page has the present-tense uses.

Mistakes English speakers make

A story told entirely in the passé composé is grammatical and reads like a list; one told entirely in the imparfait never quite happens. Most good paragraphs need both, and a plus-que-parfait only where something came before.