Reimbursable expenses

Sometimes I bill my clients for expenses incurred when doing business for them- such as marketing materials I order on their behalf. I enter it as a seperate line item on the invoice as a "Reimbursable Expense". Another line item would be for my consulting services. When I receive total payment, the total usually breaks down and matches itself to where it belongs- some dollars to the consulting fee and some to the remibursable expense. Twice lately, it is not matching properly therefore leaving me with what looks like unpaid expenses in my reimbursable expense report. I am receiving payments the same way for all clients, yet sometimes it matches properly and sometimes it doesn't. What am I missing? Thanks.