When you first get your credit card they look at your credit score, your income and maximum available credit on your revolving lines/cards. Once you have the card they are more concerned about how you use it and your consistency in paying it off. If you are using the card regular limit increase are based on your usage not your income or credit score.
When you call looking to increase your credit, but don't usually utilize your credit limit anyways, then either you are looking to take advantage of a one-time points grab (which doesn't really benefit them) or you are looking to dramatically increase your usage over a short period which is a red flag to them.
If you have a six figure income, with little debt, apply for another card with a higher limit. They will focus on your credit score and income.