The two clubs have settled on a fee for the Iceland international, but the move has stalled over personal terms.
"It seems a bit stuck," said Redknapp. "We have reached an agreement with Ajax on a transfer fee and the challenge now is that we come to a deal with the boy and that is difficult.
"We have spoken with his agent, and brother, and are not yet agreed. But it still can happen."
The two clubs are understood to have struck terms for Sigthórsson at around £7 million.
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