Sprint foot plant

Hi all

im new to the forum but have a question!
I have been constantly told that i should be running up on the balls of my feet while sprinting and although it works for me while accelerating it slows me down over a full race, this is shown when racing my group, I am infront while running flat footed (or at least what feels flat footed) but am then instructed to run back up on the toes of my feet and then fall way behind! i have been training for a year on my toes trying to improve but am still nowhere near the same speed as when i run flat footed (except for accelerating).
my question is got any tips for how to strengthen the lower leg so that i do not lose speed on my toes ? im quite short and stockily built by the way if this helps with analysis! any input much appreciated please!!!