estaban frustrando
Imperfect progressive ellos/ellas/ustedes conjugation of frustrar.

frustrar

Sí, me he dado cuenta de que los policías se estaban frustrando conmigo.
Yeah, I could tell the police were getting frustrated with me.
Esperamos cinco años esperando sacarlo, y nuestros fans se estaban frustrando mucho.
We waited for five years hoping to release it, and our fans were getting so frustrated.
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to light