FWIW, since HD does not sell the o-ring separately, some have found that HD pn#11273 which is the Viton o-ring that goes on the dowel at the base of the jugs fits perfectly or take your old one to an auto parts store and match it up to one that is acceptable for fuel.
If it is the old o-ring that was leaking it may have a little break in it. That could stop it from going on. If it is a new o-ring it could just be a little tight and need a little push. Or like stated before clean it really good. Anything in there will stop it from going back together. Good luck