getkey script to start animations???

This is getting veru old now. I wanna make my gun in my fps game move when i walk

so i make a getkey("w") command in javascript, then i make my animation, and then drag them both to the main camera...
i make the animation type 1 and change the fbx object to legacy or what ever its called.

But then some times it works and sometimes it doesnt...

Some times the animation loops over and over again even if im not pressing "w", sometimes it doesnt start at all when i press "w"
and sometimes it works

But its like 1/10 times it actuly works and when it doesnt work i must remove the animation and the model and reimport it and do it all again... but i cant really make a game when i must reimport everything all the time.

so why is this happening? why does it sometimes work and sometimes it doesnt work when im doing the same thing all the time?
plz help me... ty in advance for answer!