Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of supervisor],
I have been working on [type of work being done] for some time now, and feel that I am just not quite getting it. I don't want to continue making mistakes that I may be making or not meeting production requirements due to my lack of understanding of the work I am doing.
I would like to request additional training on my job duties. I feel like in my original training, I must have missed some vital information or misunderstood something, because my daily work is just not clicking.
I would appreciate any time you can give me with regard to this training, and feel like I could be much more productive with a little bit of help in this area.
Thank you very much,