Qantas flight vouchers restriction and failure to book on line

[PERLHR]

I had tried, various times, to book on line a flight using a QF travel voucher provided to me due to a cancelled flight to the value of 300 AUD. Despite following the instructions for booking...which is convoluted enough but workable, at the final stage the process refused to "work" despite showing prices and availability and prior to payment page, accepted the voucher details and reduced the price accordingly. On payment details it says "an error occured while creating your booking" So it was on the phone to speak to agent. After some time they confirmed availability but only more expensive by over 40 dollars . When asked why I was advised, it was due to what they see on the booking conditions their side but teh agent was rather vague.
What is it with Qantas, they are proving once again to be an absolute joke.
This price increase is nothing to do with a telephone booking fee (which I was advised was waived). . They simply seemed to indicate that the price I see on line is not valid for a credit flight voucher. That seems to me, to be totally irrational and not what should be correct. Any ideas on how this could be corrected? I feel that this should be challenged. Any idea how? The 300 dollar voucher in real terms then is only worth around 255 dollars. Anyone else encounters such issues when using a travel voucher credit?