I find the best natural tinder to be cedar bark, birch bark, milkweed pods, dry grass, dry dead tree stumps. Just remember dry and light. Think of things you find in a birds nest. A good tinder bundle should look alot like a birds nest.
Are you using a piece of flint rock or are you using a fire striking rod? If you are using a fire striking rod then it is best to use the back of a knife blade or a piece of hacksaw blade. That will give you the best spark results.