Buy a can of electronics contact cleaner with a straw tip, Q-Tips and a can of air spray with a straw tip. Unplug your laptop and disconnect the battery, put the Q-tip over the memory slot opening and drench it with contact cleaner and wipe across all the pins quickly. Change QTips and repeat until you don’t see any residue on the Q-Tip. Do this for all the slots. Wait about a minute and blast the slots with airspray.
Wait a few minutes and carefully inspect the slots for any cotton fibers and blast them away with air spray. This should remove and corrosion or foreign matter.