Something is eating my girls!

[xaote]

Hi guys/gals, thanks for taking the timeout to read my post. I am growing indoors using NFT hydro, with a few successful grows under my belt. I am 3 weeks in veg, and have noticed something is eating 1 of my 4 plants. I have spent hours trying to find the little fecker that is doing it, without success, so am unsure which treatment to try. Obviously I wanna try and get control of this before the others get attacked too.
Any suggestion peeps? It must be something pretty small (or very good at hide and seek!), but seems to eat a fair bit!
Thank you.

Edit. I am loosing small 'pieces' or section's of leaf.

[DEAN420]

any pics of the damage??

[xaote]

Several leaves of 1 plant like this.