I remember when I was taking classes, I just assumed TAs were always around. In the back of my mind, I knew TAing wasn't the only thing they did, but I grew very frustrated if they did not answer e-mail quick enough or weren't in their office outside of office hours. It was especially annoying if they didn't know all the right answers.
Its easy to assume that TAs are like teachers. All-knowing and eager to help you learn.
Just to let you know, we're students, too! We are full-time students just like you. We also have lives outside of school. We don't know all the answers. They ask us to TA courses that we took years ago.
We do want to help you learn, but we just ask for some understanding. We assign ourselves office hours to assist you, so go to them! Try not to randomly drop by outside of office hours. Email your TA. Get to know him/her.
Being at the front of a class is scary. Its like being on stage, except you have to know all the answers to the homework instead of lines.
This ramble wasn't prompted by anything my students did, but as an undergrad passing into the behind-the-scenes of TAing, I have now come to understand how hard my TAs worked for me.