Some 45,000 tickets were originally planned for Bannock-burn Live in Stirling but that was reduced to 20,000 and prices were cut.
VisitScotland has now admitted that around 9000 tickets remain unsold.
The event - which includes reenactments and performances by actors, musicians and authors - was organised to mark the 700th anniversary of the battle on June 23 and 24, 1314.
However, there has been criticism of the decision to stage National Armed Forces Day in Stirling on the same weekend, with concerns it would have an impact on the success of the historic event.
Although 20,000 tickets - costing £20 a day for adults - were made available for Bannockburn Live, only 11,000 have been snapped up.
The sales are below even VisitScotland's new target, set in April, of 15,000 tickets.
The tourism agency took charge of the event from the National Trust for Scotland at the beginning of the year amid fears over poor ticket sales.
Further concerns were raised in March, after the event was cut from three days to two and the number of tickets available scaled back to 20,000, rather than the 45,000 originally planned. Family tickets costing £60.50, including a £5.50 booking fee, were then reduced to £50.50.
A spokesman for VisitScotland, referring to their own 15,000 ticket target, said: "Over-two thirds of tickets for Bannockburn Live have been secured by the public and these are on target. Most sales are from the UK, but over a tenth are from the USA.
"We have an exceptional line-up for Bannockburn Live including talented and well-known musicians, authors, artists, clans people, living history specialists, performers, foodies and of course, warriors.
"We are on track to deliver an absolutely incredible, memorable event and we urge people to get their tickets now to avoid disappointment."
Bannockburn Live organisers raised concerns at a meeting of Holyrood's economy, enterprise and tourism committee in April over the hosting of National Armed Forces Day in Stirling on the same weekend. The free event, which commemorates men and woman serving in the UK armed forces and will take place on Saturday, was accused of taking ticket sales away from Bannockburn Live.
Peter Irvine, managing director of organisers Unique Events, said Bannockburn Live had been "truncated" after the plans for National Armed Forces Day were announced. He added: "It was a surprise when it was announced Armed Forces Day was going to be in Stirling. We did hear about it from the media."
A source close to the event told the media last month: "There is no denying everyone wanted to sell a lot more ... tickets by now.
"Considering that 45,000 were put on sale last summer, it is not ideal that sales are so low when this is such a high-profile event.
"At this stage, it is about trying to ensure the site is as busy as possible over the two days so that performers are not playing to a half-empty arena."
Around 120 news and television crews from all over the world are expected to provide global coverage of this weekend's Bannockburn Live, which will commemorate the battle between the Scots, led by Robert the Bruce, and the English.
Thousands of extra train seats are being made available for passengers heading to Stirling on Saturday and Sunday, with extra services and carriages being laid on to cope with the expected upsurge in travellers.