Vouchers query

Last i cancel flight with british airways and i apply for a voucher, total amount of that voucher it was R9478. 40...they told me every time when i want book to flight using voucher I have to call them they give voucher number. I book three trips from that voucher.. All the references that i appear on their system is for the trips that I booked using voucher... After some couple months i tried to book flight, i phone them to check my balance they told me my balance is R1118... I ask them to go through my booking nothing, they did but, their system doesn't show all the details about voucher... they said nothing they can do either i the changes where made on original ticket or voucher s were used... My concern is if changes where made or vouchers were why doesn't appear on their system