The amount of margin that you can use goes up when you add more money to your account. However, if you have a Borrowing Limit set on your account, your available margin will not increase above that limit.
If you have $2,000 cash in your account, you can invest up to $2,000 with margin. If you increase your cash account value to $3,000 by depositing $1,000, your available margin will increase to $3,000. However, if you set a $2,000 Borrowing Limit, your available margin will not go up regardless of how much you deposit.