I can't say for certain but you mentioned that you were sharing on your social media. If you were sharing on your own personal pages, that could be the cause. People that saw your first posts and wanted to subscribe, already did and those that didn't want to subscribe didn't.
So now when your posting it alerts them that your videos are up, but you aren't getting subscribers from it anymore because it's not a new audience, they've already decided whether to subscribe or not. It's still a good way to get views but you won't continue gaining many subscribers that way.
