Some engine manufacturers blocked an early lifting of the volcanic ash flying ban, the BBC has learned.
An e-mail given to the BBC as a result of a Freedom of Information Request shows that some manufacturers saw "nothing to gain" from lifting the ban.
The BBC has also been told that Air France KLM will now fully compensate stranded travellers.
The airline had previously refused to do so, but will now comply with its obligations under EU law.
'Avoid, avoid, avoid'
It was six months ago that volcanic ash sent the UK back in time.
The eruption of the Icelandic volcano, Eyjafjallajoekull, caused UK airspace to be shut down for six days.
The regulator, the Civil Aviation Authority (CAA) estimates about 33,000 flights were cancelled.
The advice for airlines and aviation authorities at the time for ash was to "avoid, avoid, avoid". There were no rules set as to what level of ash was safe to fly through.
This policy worked in other parts of the world where planes could simply fly around the affected airspace, but caused severe problems for the UK, where airspace is much more congested.
'Nothing to gain'
Crucial to lifting the flying ban was getting agreement with engine manufacturers as to what their engines could safely tolerate.
Ash can melt inside jet engines, clog the turbine blades and cause the engine to stall.
And an e-mail given to the BBC as a result of a Freedom of Information Request says: "Engine manufacturers met last night in the USA and declined to accept that their products could operate... We believe that a number of manufacturers... were supportive of the position but others saw 'nothing to gain'."
The e-mail goes on to say that if engine manufacturers had accepted the deal, all airspace could be opened.
"It did take time to get them [engine manufacturers] to understand why the particular circumstances facing Europe were unprecedented and why we needed movement from them," said the chief executive of the CAA, Andrew Haines.
However, six months on, airlines still believe the regulator could have done better.
They argue the CAA should have given them more freedom to decide whether it was safe to fly, using their own experiences and judgement.
"It just became increasingly frustrating as the days went on," said Steve Ridgway, the chief executive of Virgin Atlantic. His airline lost £30m in the crisis and saw 43,000 customers stranded.
"We started to just not believe the data that was coming out of the Met Office. Airlines were putting up aircraft on test flights... they weren't finding any ash. And it was that speed of response which became so frustrating."
Air France agreement
The CAA though argues it had to put safety first and could not lift the ban until it had agreement with engine manufacturers.
"What no airline was able to do was present us with data from their engine manufacturers - who are the experts here - which said it's safe to fly in these conditions," said Andrew Haines.
"If someone had been able to do that then we wouldn't have had this issue. But not a single airline was able to do that right the way across Europe."
There is some good news for travellers though.
One airline - the giant Air France / KLM - had been refusing to comply with its obligations under EU law to reimburse passengers for reasonable costs incurred whilst stranded.
But the CAA has told the BBC that it has just secured agreement from the airline to pay out in full.