Could be a mental thing (you think you can't hit past 20 yds, so you don't), could be lack of practice at that range, or maybe you aren't able to focus on a spot as well. I don't care for long hunting shots, but long practice shots are great for showing you how consistent you are and if you have any form problems. Little mistakes show up more at longer distances.

If you aren't grouping, then it's a consistency problem--you either aren't shooting the same way every time or you aren't focusing on the same spot every time (assuming your arrows are all the same spine and weight, and are matched to your bow).

Keep working on it, it will come together for you.

Chad