I’ve been there several times, and it’s not always as easy as “well they weren’t your bashert, so be thankful and move on!” which conbines the oft quoted attitudes of the previous replies. There is a lot of pain in uncertainty, a spectre of “is there something wrong with me?” which if allowed to fester can be very harmful. It can also make you gunshy, because you might think “I thought everything was great the last time, and look what happened!”.
In my experiences, it’s usually something simple that the other side determined to be a deal breaker; something they needed on their checklist which wasn’t met. Sometimes there are facets of a person’s personality that the other side determined would bother them too much. Not giving a reason for them is preferable to having to say something potentially insulting. There can also be an exterior reason, such as a family member acting irrational (this happens quite often) based on some hunch or intuition.
Your “radar” isn’t out of sync; men and women are extremely different, and women are often very good at concealing how they feel about something or someone. There aren’t always signs to read or cues to pick up on.