Can the Refilling Charm be used to refill potions, like the Emerald Potion in the Horcrux Cave?
In Chapter Twenty-Four of Half-Blood Prince we find the following:
“Couldn’t we make some more?” Ron asked Harry, ignoring Hermione. “It’d be great to have a stock of it.... Have a look in the book...”
Harry pulled his copy of Advanced Potion-Making out of his bag and looked up Felix Felicis.
“Blimey, it’s seriously complicated,” he said, running an eye down the list of ingredients. “And it takes six months... You’ve got to let it stew....”
Apparently, it is either impossible to use a Refilling Charm to get more of the potion, or it is possible but so far beyond the capabilities of even the top student in the school that it didn't even occur to them to try.
Of course, one could make the argument that this is something specific to Felix Felicis, since that is the particular potion mentioned in this passage, but there doesn't seem to be a particular reason why such a rule would only apply to one potion. Furthermore, we find the same thing by Polyjuice Potion in Chapter Twenty-Five of Deathly Hallows:
They remained shut in the cupboardlike room for hours at a time. Slowly the days stretched into weeks. There was problem after problem to overcome, not least of which was that their store of Polyjuice Potion was greatly depleted.
“There’s really only enough left for one of us,” said Hermione, tilting the thick mudlike potion against the lamplight.
Again, it apparently didn't occur to any of them to try a Refilling Charm.
On the other hand, the potion in the cave presents some evidence the other way. The potion was emptied when Regulus drank it, yet it was replenished by the time Dumbledore drank it. Again, after Dumbledore drank it, it was replenished by the time Voldemort went to check on it. This would seem to indicate that the potion was in some way charmed to refill.
Similarly, in this answer I presented an argument that the potion in Snape's Philosopher's Stone chamber was charmed to automatically refill.
It may be that it is technically possible to refill a potion but only the most skilled wizards such as Voldemort and Snape are actually capable of doing it. It may also be that you can't refill a potion out of nothing, but you can charm a container to refill itself from existing potion that is stored elsewhere. And it may also be that the books are mildly inconsistent.