It's very normal here that users only exchange a couple of friendly "hi, how are you?" messages and then disappear. There are many reasons for that:
1. Live gets in the way. They realise that they are simply too busy to establish regular language exchange relationships.
2. They receive too many requests and after a couple of messages choose just some selected partners with whom they want to continue.
3. They are only interested in having contact with either native English speakers or advanced English speakers and stop talking to anyone who has the same or lower level than them.
4. They got bored or find language exchange too much of a hassle and just give up.
It's difficult to find long lasting partners if you can't "offer" them something like higher proficiency, help with your native tongue (doesn't count if they're not learning your native tongue) or interesting conversation. I'm in touch with many italki users and with some on a daily basis for years, with others whenever we feel the need to discuss a certain topic. With some I correspond in English, with others in German, with others in any of my other target languages. But our goal isn't language exchange but we're friends and are mutually interested in each other. We can ask each other language related questions or correct each other but the main goal is just communication with a friend. And none of my regular friends sends me "hi, how are you?" messages but we always dive right into some topics we want to discuss ("Have you read this? What's your opinion about that?").
How about you write something in your profile about your hobbies or topics you're passionate about? If you find like-minded people, then it's easier to correspond regularly. Users who are active in the forum receive more language partner requests. Start interesting discussion topics and people will take notice of you and want to connect with you.
Though I'm not a native English speaker, I hope it's ok, if I give you some suggestions:
What is the main reason why many people do not reply to messages when you contact them for improving your target languages?
My question is: Did it happen to you and what did you do then?
briefly