Amendment of I 140

I need some suggestions regarding the amendment of priority date in I-140. In my I-140 receipt notice, the priority date is April 2015. Last year when my employer gave me I-140 approval copy, I have noticed that in my I-140 approval, the priority date is December 2015. I have checked with my employer and they are trying to do the amendment, but USCIS rejected the request. Has anyone faced this kind of issue and has anyone successfully amended I-140? Thanks!