Often something was spilled into the keyboard. You will need to clean the keyboard or replace it.
I would start by taking of the keycaps and then carefully clean things up. I would use distilled water (not tap water!) if you spilled anything sticky. Use a cotton swab (Q-Tip) slightly damp (not wet!). You should then go over the ares again with a high grade (85% or better) Isopropyl alcohol again with a damp cotton swab. While that might fix things its not getting into the deeper areas of the key. That gets into having to completely taking the system apart.
Follow this IFIXIT guide: [guide|6040|MacBook Pro 13" Unibody Mid 2009 Upper Case Replacement]. Now the tricky part even here its not easy! as you will need to remove 85 + snap refits or screws to take the keyboard out. I don't even try any longer as I've found more often than not the contacts are to far gone to fix. I just replace the upper case assembly which is what I recommend. While more expensive in the long haul it will be cheaper! Here's the IFIXIT part: [product|IF163-000|MacBook Pro 13" Unibody (Mid 2009/Mid 2010) Upper Case].