The three candidates went head to head at Dudley Town Hall on Monday night in a debate screened on Channel 4.
Ms Long-Bailey, who is seen as the most closely aligned politically to Jeremy Corbyn and is backed by many of his supporters, insisted Brexit was ultimately why Labour lost the 2019 election.
She said the way Labour's manifesto was "packaged" confused voters but that the party "shouldn't throw away the policies".
WATCH the full debate in Dudley here:
Centrist candidate Lisa Nandy, however, said Mr Corbyn was an unpopular leader and that Labour must now grasp the opportunity to change direction.
The debate was held in the constituency of Dudley North, which was among the Labour 'red wall' seats to fall to the Tories.
But the candidates differed on whether Brexit or Mr Corbyn was ultimately to blame.
Ms Long-Bailey said: "We suffered one of the most devastating defeats that we've seen in 100 years and we lost seats like this in Dudley North and right across the West Midlands. And there were many reasons for this defeat, 52 of the 54 seats were Leave seats but we also need to recognise the other myriad of reasons why our voters lost trust in us.
"We had transformational promises in our manifesto but no message that brought them all together."
Ms Nandy, who revealed she would vote to abolish the monarchy in a future referendum, said: "2019 was no ordinary election. Dudley, Sandwell, Stoke, Mansfield, Bolsover, Gedling, these are places where Labour runs through people's DNA as much as their family history and the fact we got to the point where we were going round doors with people saying "not you, not now, not this time", that we had people voting Tory in order to tell us that we had to change, that was a real moment for us.
"We had an unpopular leader, that was true, and we had an unpopular leadership team."
Sir Keir Starmer remains the bookies' favourite to become the next Labour leader.