Why was confederate diplomacy toward great britain unsuccessful?

Mostly because they believed that Britain needed them. They relied on Britain getting cotton and other resources from them while in reality the British didn't need it as much, which meant that their plans were wrong from the start and that they would eventually fail economically. This is why their economy started failing and their money started losing value so they couldn't manufacture things and they couldn't sell them which meant that they couldn't sustain the war effort and and couldn't sustain their struggle. Eventually they lost the civil war because of things like this.

One of the reasons confederate diplomacy toward Great Britain failed was because Britain was not as dependent on cotton grown in the south as the Confederacy had thought. The Confederacy originally believed they would be able to hold Britain's need for cotton over their heads as a threat, but this plan did not pan out.