Did you know that good singers are also good listeners? In a choir or group, they listen for the harmonies of the other parts and try to blend well with them. They listen closely to each pitch so they wont go flat or sharp. They also listen to their instructor or coach when suggestions are made to improve. And they remember the instruction as they are singing. If someone has been singing incorrectly (such as not opening their mouth or their diction is incorrect), it takes practice to fix that, just as with any other bad habit. Many times, I say the same things over and over again to my students because they do not remember my past instructions or they are not concentrating. So, try to concentrate and remember the suggestions you receive and put them to use.