The Allam family had been confident of selling the Championship club before Christmas after agreeing a price with a group of Saudi investors.
Proof of funds had also been provided and the deal, which has been in place since late November, was expected to go through without a hitch.
However, The Yorkshire Post understands one member of the consortium, a property developer, is seeking further clarification on possible future development of the site that houses the KCOM Stadium before pushing ahead and completing the sale.
The Allam family, whose eighth anniversary of buying the East Riding club is today, are not closing the door on Duffen’s group reviving their now stalled takeover.
But, talks are understood to have taken place in recent days with other interested parties.
These include a US group of businessman and Sportyco, a crowd-funding organisation linked with Hull City Supporters Trust, who had both previously met the Allams to discuss a possibly buying the Tigers.