Laura turn'd cold as stone To find her sister heard that cry alone, That goblin cry, "Come buy our fruits, come buy." Must she then buy no more such dainty fruit? Must she no more such succous pasture find, Gone deaf and blind?
Laura freaks out when Lizzie tells her that she can hear the goblins. Why can Lizzie hear them, while she can't? Does that mean she won't be able to eat anymore of that tasty, tasty goblin fruit?
It's still not clear why Laura can't hear the goblins anymore, and it's probably supposed to stay a mystery. Lines 257-258 are phrased as questions, so if you're wondering what's going on, don't worry: you're supposed to.
Her tree of life droop'd from the root: She said not one word in her heart's sore ache; But peering thro' the dimness, nought discerning, Trudg'd home, her pitcher dripping all the way; So crept to bed, and lay Silent till Lizzie slept; Then sat up in a passionate yearning, And gnash'd her teeth for baulk'd desire, and wept As if her heart would break.
Laura's so depressed when she finds out that she's been cut off from that delicious, drug-like goblin fruit that she gets all weak—her "tree of life" (AKA her health) gets droopy.
Laura doesn't say anything to Lizzie about what's upsetting her, she just "trudges" home and goes straight to bed and sulks, like any angsty teenager might do.
But after Lizzie's asleep, Laura sits up in bed – she's jonesing pretty hard for that goblin fruit.
The "desire" she feels for the goblin fruit is described in almost erotic terms – her "passionate yearning" and "baulked" (i.e., unsatisfied) "desire."
Laura cries and cries, and doesn't sleep.
Day after day, night after night, Laura kept watch in vain In sullen silence of exceeding pain. She never caught again the goblin cry: "Come buy, come buy;" - She never spied the goblin men Hawking their fruits along the glen: But when the noon wax'd bright Her hair grew thin and grey; She dwindled, as the fair full moon doth turn To swift decay and burn Her fire away.
Time passes. It's not clear how much time – it's just "day after day, night after night."
Laura still yearns for the goblin fruit, and still can't even hear the goblin men as they pass.
The "sullen silence" of line 271 could have a double meaning. Laura can't hear the goblin men, and everything is "silent" around her, and she's "silent" herself, in that she hasn't told Lizzie what the problem is.
Even though she "kept watch," Laura can't hear or see the goblin men.
By the time of the next full moon (i.e., "when the moon waxed bright"), Laura's hair suddenly goes gray.
Apparently, eating the goblin fruit somehow tied Laura's life to the moon, so that by the time the moon wanes away, Laura will die.