Is it natural to say:
我不知道你为什么不常回我信息 (I don't know why you take so long to text me back).
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Sign up to join this communityIs it natural to say:
我不知道你为什么不常回我信息 (I don't know why you take so long to text me back).
Your sentence is fine. 我不知道你为什么不常回我信息. However, your English translation doesn't match with your Chinese sentence.
I don't know why you take so long to text me back.
我不知道你为什么不常回我信息. means I don't know why you don't text me back usually.
The neat way to say "why you don't text me back usually":
你怎么总不回我信息呢?
你怎么老不回我信息呢?
你为啥经常不回我信息?
你为啥总不回我信息?
The way to say "I don't know why you take so long to text me back":
你为什么信息回得这么慢!
the one like Pedroski suggested: 我想不通为什么你回复我的信息要这么久!
I would write something like:
我想不通为什么你回复我的信息要这么久!