This attitude occurs many times from people who ask for friendship and then seem just wanting to have more people on their friend's list.
The strange thing is however, that you say all of your friends show the same attitude. Maybe you should keep looking for other friends and try to talk to them before adding them to see how serious they are about exchanging languages.
Honestly , I encountered such members few times, but other members on the other hand were very keen to help me and improve their level as well.
Maybe the time difference might be a problem as well, so you should consider setting appropriate times for meeting your Chinese partners.
I hope this will help you find suitable partners for mutual benefit.