ANA could be effective, when they made guerilla at the very start of the war, they even saved the republic by this way. But republicans wanted to controle the ANA, and didn't want them to be effective, so the guerilla was forbidden (death punished) and the supply was only allowed to the bourgeois units.
Later there will be a "guerilla" military force organised by communists, but it will be not as effective, and very too late.
Left republicans prefered to risk the loss of the war than to share a victory with revolutionnaries.
"Our government or the Franco power; Durruti: never ever ever."
Republicans prefer facism than democracy.