First, he had to call off his overland campaign because his supply lines were cut by Nathan Bedford Forrest. Then, Grant sent Sherman on a failed expedition where the Union absorbed heavy losses from a direct assault. Grant followed up by attempting to navigate alternate courses through bayous and canals - the terrain beat him. And then he tried another direct assault and that failed miserably. He finally opted for a seige. Luckily, he had the manpower and resources so that his multiple failures didn't end him. It took him approximately 9 months of failed manuevers before he successfully took Vicksburg.