Hi all
My first brew has matured enough to taste and it's flat

I think I know what I've done wrong but I thought I'd check. I added the sugar and it seems to have fermented away. However as I didn't have any petroleum jelly I didn't smear any on the barrel-top thread as advised. Also I don't have a barrel-top spanner so merely tightened it as much as could by hand.
Is this what I have done wrong? It really is very flat - no life at all.