AR not locking bolt

Hello. I have an AR that I am having some trouble with and I am wondering if I need a heavier buffer or a stiffer spring. If I manually load a bullet and drop the bolt it will slam closed and I am able to fire. However when I shoot the bolt will not close all the way (doesn't grab the back of the bullet fully) and the bullet gets jammed and I cannot pull the bolt out unless I bang the buttstock on a table while also pulling the charging handle. I am thinking I need more force behind the buffer to get the bolt all the way closes. Where should I start? Stiffer spring or heavier buffer?