Flying lines (the long thin lines) dont stretch but shrink over time.
The frontlines, in general, face more load during use than the backlines and hence, the backlines shrink more than the frontlines.
This uneven length may enhance oversheeting and stall. Its adviced to check and, if needed, re-calibrate your bar each few months.
Below link shows how to adjust via the trimline, there is more than enough, but you can also adjust by applying a knot in your pigtails.

Bar calibration