Can't hit/mine anything while sprinting/walking

I can't seem to hit any blocks or entities while sprinting or walking!
What I expected was to hit players and mine blocks while I was running!
However, what actually happened was my sword wasn't moving while running, just sitting in my hand, even though I left-clicked like crazy!
I dunno if this is a bug or my Acer computer having a *****, but I lose PVP cos of this!!!
I couldn't find this in the Navigator!!!
To catch this bug:
1. Open a world
2. Sprint and try to hit a block or animal, it doesn't work for me!!