I stretch my line almost every time I use it and every time it is cleaned or dressed. If it is kinky off the package spool it will take a bit of work to spin the kinks=twists out of it. Start by pulling off 50' or so of the line and stretching away from the reel a few feet at a time and getting the kinks to spin out at the leader end- if you start at the leader the kinks will end up on your reel.
Once most of the kinks are worked out, the line can be tied between two bushes (or what ever) and you can walk the line with a damp soapy rag to remove dirt/algae/old dressing/etc. Gripping the line firmly will further stretch it, repeat with dressing, then repeat with dry clean rag to remove excess dressing. (excessive dressing will pick up dirt) I clean the lines a couple times a year.
others stretch lines in other ways
Almost guaranteed that a cracked coating is allowing water to soak into the core is the reason it sinks.