Melons don't appear in my inventory when I harvest them

just what's in the summary line: I mine a melon with an axe; I see the little melons indicating that I can pick them up; I walk over there and hear the popping sound I normally associate with picking them up, but when I look in my inventory - there's nothing there. 

I restarted my computer, but the problem remains. 

I am playing on a realm.