Sounds like you might not be using a cheese thats suited for melting if you're having a tough time. You want something with high moisture and fat content. Cheddar, Havarti, Mozzarella, Gruyere, or Monterey Jack should work. You can use shredded bag cheeses but make sure the bag says its a melting cheese, as regular bagged cheese has anti clumping agents that fuck up the melt.