Everybody hates them. Everybody feels theirs could be more flexible. Everybody is probably right unfortunately! The hamstring muscles are notorious for being tight, because of their location crossing both the hip and knee joints.
In order to fully stretch them you must have the hip and knee fully extended, and hold your stretch! This is a rare position to be in day-to-day and almost all sports and activities involve flexing these joints, so the hamstrings rarely experience full stretch.