Wi-Fi Not Working

I seem to be having a problem with getting my MacBook Pro on the Internet from my bedroom. The router is on the ground floor in my house and my bedroom is on the top floor. However, I know the problem I'm having isn't anything to do with the signal because I go on the Internet from my bedroom all the time and it just gives me trouble at random times. I have no problems on my old Dell computer or on my iPad with getting Internet from my room. I know there isn't anything wrong with my Mac itself because I take it to school every day and the Internet there works just fine. I also know it's not a case of how many computers my router can handle either because when I take my Mac downstairs and use it, it works just fine. What is the problem and how do I sort it out?