"When I started, I received commission from deals booked by previous employees where I took over the account, so that's whats worrying me....."
It seems to me that you are right to be worried. If the employment contract is not explicit on the issue, we look at what is implied. What is implied by you having got these commissions is that the commission goes not to the employee who books the deal, but the employee who books the payment. If sales have gone up during your period, this will be harsh on you; but if sales are fairly static, then it would be wrong (in my opinion) for you to expect payment.