What advantage was the Confederacy missing that might have turned the war in its favor?

They were missing official recognition by the British and French, which might have happened, until Lincoln skillfully deflected it by issuing the Emancipation Proclamation.

The Confederacy was missing a number of important things and even so, it still had a chance.

• The South was missing more of an industrial base than it had;
• An alliance with Great Britain would have let the South go "free". Depending of course on the degree of commitment.
• A larger civilian population with which to bring into the army.