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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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":
The way to say "I don't know why you take so long to text me back":
the one like Pedroski suggested: 我想不通为什么你回复我的信息要这么久！