Why can I always smell smoke in my room?

you should check all your plug sockets and wires to make sure none of them are damaged.

and old occupants in the house can leave bad memorys , thoughts and smells !!

If you live in an apt. complex cigarette or cigar smoke can travel through the vents and even electric sockets. However, if this is not the same smell as cigarettes or cigars etc. check all your appliances for frayed wires or overloaded surge suppressors and even your outlets may give off an odor of something like burning plastic. Also check the walls for hot spots near outlets. Also another indication of a problem is if your outlet box keeps shutting off. I would unplug everything and see if this is or isn't the cause of the smell.Also I'd get the landlord to check the outlets and such for shorts.If you're living near a bar or a spot where employees go out to smoke,this may be where you're getting the smoke smell as smoke can travel even outside. Also check if people once in a while smoke near the entrance of your building or around the grounds near your room. Peace.

it could be becuase of the air conditioning

1. Get your house checked for carbon monoxide.
2. Get out of that rooom.
3. maybe the people who lived there before were smokers

Maybe the people who lived there before did.
Do you have carpet? it might keep the smell.
Also the air bents could move the smell around.
Does anyone else smokes in your house?
Have a clean up and use lots of Fabreze

Good Luck