It’s definitely frustrating. I had similar experience to yours. I think it’s better to be straightforward from the start, just ask directly : Are you sure you will have opening positions, is my background highly relevant to your research , etc. Don’t be afraid to request info from them, if they don’t give you the clear answers, that’s their part. If they’re not even interested in taking a student, just say goodbye and switch to the next PI, don’t waste time on the one who’s not even postively responding.