According to a recent product update from Pinterest they are now only pushing out story pins that have a good engagement right from the start. This might be what is causing a drop in impressions for your latest pins. Here’s a detailed update from Pinterest itself on this topic:https://community.pinterest.biz/t5/product-faq/product-release-roundup-for-creators-3-17/ta-p/14262
We would recommend to pin more regular image pins and fewer story pins and focus on creating content that drives a high click-through rate or engagement. Once your engagement is back up you can start creating more story pins. This will help your account grow no matter what format of pins you use.