I have two droplets on the Singapore DC, both of which don't allow for Fast Resize.
This is a pain for me, because I have webuzo cPanel and SSL keys on both, which all need to be reconfigured if I want to load them from a snapshot.
The Fast Resize feature is limited to the resources available on the physical hypervisor that your server is on. If there are no resources available, you will not be able to resize your droplet, unfortunately.
You can take a snapshot of your droplet, delete your droplet to release the IP, and once it's fully deleted, spin up a new droplet with the snapshot you took. The new droplet will almost always have the same IP address.
It depends on the hypervisor your droplet is on. If it it doesn't have enough space for an upgraded droplet, you will not be able to upgrade your droplet. But you can take a snapshot and re-deploy it so that it's deployed on a different hypervisor.