Breakdown of En el expediente de la beca faltaba una fotocopia, así que tuve que posponer la cita.
Questions & Answers about En el expediente de la beca faltaba una fotocopia, así que tuve que posponer la cita.
Because faltaba is the imperfect, and it presents the missing photocopy as a background situation or an ongoing state at that moment.
- Faltaba una fotocopia = a photocopy was missing
- It describes the situation in the file when the speaker discovered the problem.
If you said faltó una fotocopia, it would sound more like a completed event: a photocopy was missing / failed to be there at a specific moment, which is less natural here.
In this sentence, the logic is:
- first, there was a situation: a photocopy was missing
- because of that, a completed action happened: tuve que posponer la cita
So the combination faltaba + tuve que is very natural.