I've had good luck with WD-40 for this kind of stuff. If you spray a bit in a cup, dip a cotton swab in it, then blot off the excess with a cloth or paper towel and apply it sparingly to the exposed cog and pinion gears on the back of each tuner. Work it in afterwards by turning each tuner back and forth.
If that doesn't work, they might be binding up in the sleeves that hold the tuning key shafts on, or between the posts and ferrules up on top of the headstock. You could carefully try to get a bit of the lubricant in the gaps where those parts meet (maybe dip a toothpick in the WD-40 and use it to apply). You don't want that stuff soaking into the wood, though - so just use trace amounts. Optimally, pulling them and cleaning/lubing them off the bass would probably have the best outcome if it gets to this point.
