New learners: a much more natural way to put this sentence would be "这三天(以来)你怎么不回我(的)电话(呢)?" Keeping as close as possible to Duolingo's suggested sentence, 为什么 (or 怎么) would be better put directly after the 你.
"Why did you not return my calls during the past three days?" should be accepted. The official answer uses "the past three days" but Duolingo wants "these past three days" for this particular variation for no apparent reason.