I know it must be hard! ;^;
I have a good explanation to why artists might not accept trades sometimes, but i'll keep it under spoilers because it's quite long;
I used to wonder why wouldn't people with really nice art trade with other people with less experience, but consider:
Most artists who have really good quality drawings and a good grasp on the art fundamentals either work with art full-time, or have real life jobs and other things that keep them busy (i'm saying this because a really experienced artist takes years to learn, so you probably won't find anyone that's a high level pro at a super young age, and even the youngest ones, like, 15 might be very busy with school, etc). So, they need to optimize their time.
A lot of artists won't even trade with anyone unless friends, because they need the time to spend on things that are more useful to them (like selling commissions), something they can benefit from in real life.
Even the ones who do trade have reason for that. You see, a high skilled artist takes hours to finish a drawing, since they put a lot of quality and effort to it. And a drawing from someone less skilled will often take less time, and come out with less quality than their art. So, i know it feels upsetting to not be accepted in a trade with some artists, but keep in mind it also might feel really frustrating to them to spend hours, maybe days on a drawing they are doing pratically for free, getting in the middle of their commission schedule, or even job/real life schedule.
For some of them, it might feel like playing with someone at a younger age Like a 20 years old person playing with a 10.
I, myself, tend to refuse art trades when i'm busy or overloaded with commissions so it doesn't get in the way of my agenda.
Also, a little secret, something i found out that actually works is to contact multiple artists which you admire and ask them for trades. Be sure to be nice and explain why you want to trade/how long you've been admiring their art for, and that you don't mean to bother them or get in the way of their schedule, if they don't have the time to do it. It usually works, and at least one of them tends to accept the trade. You get a bigger chance. Don't give up!
Last of all is to never compare yourself to other people, as Fizz said, or dismiss other people because you think they're at an inferior level.