First blow or suck all the debris from the keyboard. If this doesn't do it then.
Get the following supplies:
1. Q tip cotton swabs
2. Alcohol ( type you use to clean cuts)
3. Small flat sturdy instrument.
Write down the placement of the four keys that surrond the Fn key.
Slowly pry the function key a little bit form one side and then the other until key pops off. Do not lift the other keys yet.
moisten the q tip ( is should not be so full that alcohol can be squeezed out, just wetted)
Next clean around the button with both sides of the q tip.
Is it cleaning any goop? can you clean all around the key?
If you need more room remove the necessary keys.
Does it help?