First off, what are you willing to count as a fair declaration for a fate point? In my group, a free aspect on the scene is always fair, the arrival of a missing PC is always fair, and that plus a semi-plausible story/excuse in exchange for the player's otherwise unimpeded concession is always fair. These are not all that declarations can do, but they form the rock solid bedrock that my players can rely on. On the other hand, I've been in games where I couldn't get an easy aspect on anything, and I've also been in games when a single fate point tagging an opponents consequence was enough to force a concession from the bad guys.
In your case, it sounds like the players don't think a declaration is worth enough. Your party might not need my kind of declarations often however- Lots of games have it as a given that fights start with the whole party present, and few games assume the players are going to walk away from fights they lost. Make a list of a few cool things you would do with declarations, and then point them out to your players next time it seems useful.
Second, I suggest eating your own dogfood. That is, start giving the antagonists fate points, and using them gleefully. Give them a fair amount (offhand, I'd suggest an encounter having about half the fate points of the PCs, but that is an extremely fuzzy relationship) and play hardball. However the rest of your DMing style is, when it comes to the foes fate points always use them to do your level best to do the players in. See how it goes.
This feels weird, I know- the DM paying a fate point to say something's here? Isn't that just what the DM does? But your description of the battlefield isn't usually slanted- the first time you spend a fate point to have a villain rush to a conveniently placed balista, there may be more than a little grumbling. As soon as that happens, capitalize! "Well, fair's fair. If you were to declare that there was another on your side of the room, intended to give a complete range of fire, and maybe hand me one of those shiny fate points..." They'll catch on- and one of them may start grinning evilly every time you hand out the new lava moat generators- Er, I mean fresh fate points.
But the opposite may happen. You may, while trying to spend your villains fate points, find that it is just not effective to spend them on declarations. You limit yourself too much, you just can't seem to take advantage of it, or maybe that reroll is just far to useful to pass up. You have now learned something important. Namely, you need to start letting bigger things get declared, or accept that declarations are not often going to take place in your game.
Lastly, and less reliably, let them be used more freely on NPCs. When a fate point is spent on an NPC buddy to have a certain skill, I let them keep it. Some of the best loved sidekicks and buddies have developed all sorts of odd character quirks- Joe the Armorsmith has a Knowledge Nobility 8, after the party spent a few fate points to get information once, and after that realized they could make him better easier than anyone could take ranks in it. He's a valued informant, and a bit more human (well, dwarven, but you know) with that unexpected touch.
The cute barista at the coffee shop that had barely been flavor before keeps an assault rifle under the counter, and it's loaded with silver. We're still not sure why, but the players are curious as heck to know, especially the player that spent three fate points to declare an armed Guns 3 ally show up when the loup-garou (werewolf, for those that don't speak french) attacked. I sure don't know. I've got some theories, but I'm curious to see if anyone declares an answer I didn't think of.