Sounds like they're not interested, or have too much baggage. People with problems are always *****ing and being negative, because there's always problems on their mind. Avoid.
I suspect low interest if they're not reciprocating yours, however, or showing any signs themselves. They shouldn't just be complaining - unless they're not interested. Then it's just like dumping all that **** on any average stranger willing to listen. ESPECIALLY if you're flirting and showing interest, and they remain closed off, non-chalant, *****y, and complaining about everything... Like they would to a FRIEND.
I used to do this with girls I wasn't interested in, and vice versa. Usually *****ing about other women, or them with men. A girl that's interested will be happy, flirtacious, wanting to know you, looking into your eyes, complimenting you, etc, etc - regardless of her problems. She probably won't even bring them up - out of fear of turning you off, and so forth. Even if you're an easy-going listener. That just attracts them more, and adds to your good qualities. Or when being shy is "cute".
Don't waste time with low-interest girls.