Sprinting Stops half way though a block jump.

Hi,
I recently started doing a parkour map. There are alot of 3,4 block jumps. etc etc..

So i come to a coupel of jumps, i sprint, (double tapping W, then holding W) my sprint goes, then half way though the block the sprint stops and goes to a normal walking pace..

So i decided to try again.. (double tapping W, then holding W) But this time i held the W key even harder makign sure i don't let go.. i never let go and my sprint fails half way though the jump.. It just goes to a normal walking pace.