Jib leads and leech cords
This sounds like either a leech cord or a jib lead problem. Either way, it's a simple fix.
If tightening the leech cord (cable) inside the leech (the sail's back edge) doesn't do the job, the jib sheet's too far aft. Move the block (called the lead -- "leed") forward. You'll notice that the jib leech straightens.
The best way to gauge that the lead is set properly is to have three pairs of telltales (lengths of yarn or ribbon) evenly distributed near the luff (forward edge) of the jib. When the lead's set right, the three telltails on the windward side of the jib should all be doing the same thing at about the same time. If you're trimmed correctly, they'll all be streaming straight aft simultaneously. On most jibs, when the lead is colrrect for sailing to windward, it's too far aft for reaching.
Luff telltales don't work on a mainsail because the air flow there is disturned by the mast.