The SM58 has been a great workhorse of the vocal world, HOWEVER, you must almost swallow it to get any reaction from it. At greater than 1 inch away, the gain pickup drops about 50% unless you're willing to yell into it. I've never liked them.
The SM58 Beta is better. I would try ckobu's advice and if you're still having issues, look at a different mic. AKG and Electrovoice make good mics.