For the time zone difference, you will just have to deal with it, by choosing practice times during your mornings or nights. Weekends are better than weekdays. Also, apps like Calendly (which is free, by the way) can help with choosing a time, because it does the time zone conversion automatically by showing the user's local time, i.e. - it will say 9 AM Beijing time for you and 9 PM New York time for a partner in NYC, so you don't have to calculate the difference.
Although a few native speakers of English are learning Chinese for career reasons, many of them are learning it as a hobby. That means, many other obligations in life (work, taking care of kids, going to the gym, school, etc) are more important than their hobby of learning Chinese. You need to understand this and not push too hard. The harder you push, the more you will annoy someone and then they will stop talking to you.
Also, you should try to become their friend, instead of "study partner." The people whom I've talked to for the longest time on italki are all my friends. We feel comfortable with each other, we have a chemistry, we share a similar sense of humor, we have common hobbies and interests, and it never feels like a chore or study session when we are practicing languages. It just feels like catching up with a friend.
Finding a good language partner is often random and unpredictable. Some weeks, I meet 2-3 people on italki whom I have great conversations with, and I don't have time to talk to all of them. Other times, I will meet no one interesting for weeks. You just have to accept that and keep trying, a little every week.
Also, use a profile picture that shows your face with a bright smile in the best way possible. This is not a dating site, but it's just human nature to reply to more attractive people. If you use a better profile picture, you will receive a lot more messages and people (guys and girls) will be nicer to you.
Finally, if you don't want to deal with the complexities of language exchange partners, pay for a teacher (community tutor or professional teacher) for English lessons. He/she will show up on time, every time, that you want.
