Betting as you fatigued an anchor point changed ever so slightly...but at 60yds even a slight 1/16 or smaller change in anchor can have the results you described...
At short shot distances... under 20 yds to 10 simply use 20 pin under X instead of dead on...with most modern bows of today in that 60-70 lb the 20 pin will keep you in a kill zone
Here's where it gets interesting...at 5 yards use your 40 yd pin...yup FORTY YARD PIN...lol...try it!!!
Neil