If you create an expense entry by entering a credit card charge or link a credit card account to an existing expense entry, Core unchecks and disables the Reimbursable flag for such expenses. This is because when you link an expense to a credit card account, it implies that the employee paid off the expense using a company credit card. Such expenses are not to be reimbursed to the employee. All you need to do is pay the credit card bills via Checks. Check the Core Help for more on using credit cards.