If you mean that you have trouble getting the chain to get into the big ring (but it shifts ok on the rear gears) then there are 2 likely options. One is that the front derailleur cable is too loose. You can tighten it at the barrel adjuster on the downtube by turning it counterclockwise. Try one partial turn (there are knobs on the barrel) at at time. If after a few turns this doesn't fix the problem, then the upper limit needs to be examined. You can look up instructions on the Park Tools website or go to the LBS for an adjustment. Note that often it will shift ok on a stand (at the LBS) but has trouble under load (when you're on it), so it's nice to teach yourself how to do this. Do not loosen (or tighten, but in this case you'd loosen it) the limit more than about 1/8 of a turn at a time.

Also, cables do not stretch. If they are truly stretching, then they are probably fraying and/or slipping and are about to blow. This happens at the end of cable life, not at the beginning. Symptoms might be repeated spontaneous loss of cable tension. What happens with new cables is that they become more firmly seated in the housing over the first several rides just due to normal tension after installation. If you have persistent problems with "cable stretch" symptoms, then you might want to remove tension from the cable by shifting into the smallest ring and cog and loosening the barrel adjusters and the cable at the pinch bolt, then yank sideways fairly hard on the problem cable where it runs along the underneath of the downtube. Use a fourth hand tool or clamp to re-tighten the cable tension at least most of the way and then re-index (get proper tightness) using the barrel adjusters.