The majority of people, including truly born-again Christians probably have evil spirits speaking to them, often in the first person, so people think it’s their own thoughts. But they’re not originating these thoughts. They’re thoughts we hear. They’re demons speaking.
If we ask God a question and get more than one answer, only one voice can be the Holy Spirit. The demons need to be excised.
I discovered this in the early ’80s, having been being born-again in ’74, and haven’t had the problem since.
Not being truly born-again can also be a reason, but may not be the main reason. Many have been truly born-again and experience confusion when trying to hear God’s voice, like I did in the early ’80s, when God showed me what actually was going on.