Melee while sprinting ? Why doesn't it work?

Hey guys I got a little problem in SvM.

It happened to me now multiple times that I sprinted towards a Merc from behind and wanted to melee him. I pressed the melee button and all that happened was me sprinting past him and getting killed. One time he even missed me, I threw a flash and came from the side. I spammed the melee key and nothing at all did happen. I ran into the Merc, and my Spy didn't melee.

Does anybody have an idea why I can't melee a Merc?

Please help.