[Fizzlemizz]

Where you're setting the OnClick for each button, you're using a fixed string to print.

print(buttonName)
Where is buttonName defined? If it's global and being overwritten before you get to click the that's what it's value will be. If it's local inside a chunk the OnClick handlers can't "see" then it will be nil or???

local index = 0
while true do
    index = index + 1
    local buttonName = "PetJournalListScrollFrameButton" .. index
    print(buttonName) -- debug --
    local button = _G[buttonName]
    if not button then break end
    button:HookScript("OnClick", function(self)
        print(self:GetName()) -- debug --
    end)
end

Seems to work.