From your description it is a good possibility that the luff boltrope needs to be 'eased' or adjusted (by a sailmaker). Boltropes change dimension over time due to the constant stretching (they get progressively shorter over time) and that leads to increased draft, draft aft and with a hooked up leech. Also, a boltroped sail unless otherwise specified when built will be designed for ~15kts. of wind strength and if you sail at ~5kts the luff section will be too 'rounded' ... and you will easily get so-called 'backwind' because of the deep draft shape.
Go to How to properly RAISE a woven dacron mainsail - SailboatOwners.com
and go to the section on checking the boltrope near the end of the article by checking the angle that the boom makes with the mast when the sail is raised, etc. If that angle isnt 'right' then take the sail to a sailmaker and have the boltrope 'eased' .... pretty cheap alteration. Note - a woven dacron mainsail that is typically flown/used in 'light' winds should have less 'boltrope preload' so that the luff entry shape is more or less FLAT.
The most common cause of shape distortion in a woven dacron mainsail is not 'stretched out' but rather a 'shrunken boltrope' ...... OR the sail isnt being properly 'stretched out' with proper halyard tension when being raised.
hope this helps.