Improve the bracketed part of the sentence.
We were still waiting in the queue for tickets when the show (had begun).

When two actions happen in the past, one after another or simultaneously disrupting an ongoing action, we use the Past Continuous Tense for the ongoing action ("were still waiting") and the Simple Past Tense for the action that occurs or interrupts it.
Therefore, "had begun" (Past Perfect) should be replaced with "began" (Simple Past).