It sounds like either there is something stuck in them, food, debris, stickiness due to liquid, etc., or the keys are becoming damaged due to over use.
First I would suggest you try cleaning out the keyboard. Anything stuck in it can cause keys to stick. And even the smallest amount of a liquid that can turn sticky (basically anything other than water) can cause keys to get stuck.
If cleaning it out doesn't do the trick, then you are looking at either replacing the keys (if possible on that laptop keyboard), replacing the whole keyboard or using an external keyboard.
Thanks for replying, I don't think that it will be dirt as it has been happening since i got the laptop. I will clean it anyway. I'm not worried too much if the problem stays anyway as I've gotten used to it. Again thanks for replying