Do you mean, locust or lotus? If it’s locust… has a lot to do with core, back strength. You might also want to bring more weight into your shoulders (yes you need some good flexibility in your shoulders. They will open up with the practice too). It is much easier to come up with the legs slightly bent and feet separated. I don’t recommend it though. Just means your core is not strong enough and you are mainly not ready to do the pose. My main focus was to go as high with my legs super straight. Worked my alignment and my strength. When I got strong enough, I did lift up. Takes a while.
Since when have you started doing the advance class. I imagine you have a great regular practice to do the advance after one year!
Don’t push so hard. Your body adapts very quickly and the strength will come with practice. Like my teacher said, you’d rather do a posture gracefully with great control than doing it all wrong. You don’t want to hurt yourself. If you do not feel ready to go up then work of every little details to go up.
For your core strength it’s mostly the transverse of the abdomen and the multifidus that will help you. They are deeper muscles compared to the abs and back extensors that are more superficial. I’ll post a link about it when I get the chance. Theses muscles are much harder to contract but are responsible for most of the core strength. I still have a hard time using them in most of my postures.
Good luck!
