Shocking my well: vent draining extremely slowly

I'm trying to shock my well. After removing the vent plug to pour the water/bleach mixture, I realized I need to add water veeeeery slowly for it not to overflow (~1gal/20 min). I put a stick through the hole, felt a squishy layer of about 1/3" and then something hard. Does anyone know what's going on and what should I do? Thanks!