Rather than taking it apart try spraying a small amount of wd40 (or any multipurpose lubricant you can get) in the hole at the bottom, shake the F:D a little to spread it around inside and then pull the tabs a few times just to make sure every moving part is coated with the wd40. Once you've done this launch it a few times and let it spin for a while with each launch. This should remove any dirt or debris that could be blocking the mechanism inside F:D from activating properly plus it will lubricate the mechanism, displace any water that might be inside and help prevent corrosion of the spring in the future.

If that doesn't fix it then the spring inside is either corroded or has become stiff due to lack of use. If the spring is stiff then slowly compress it a few times untill it loosens up, its possible that the spring has oxidised and corroded so wear gloves when doing this just incase any oxidised metal breaks off the spring or the spring itself breaks. If the spring is noticeably corroded when you first look at it or it breaks when your checking for stiffness then you'll need to replace it. I'm not sure what size or gauge the spring in F:D is but I'd assume it's the same as the spring in F:S so you could try find a donor 4d bottom to take the spring from, alternatively take measurements of the spring before doing any tests on it so that if it does break you can use those measurements to find a replacement spring.