He told us that it was both technically flawed (the choice not to use graphite as a moderator because no one wanted to challenge a wrong result calculated by a famous German physicist on the nuclear cross section of graphite. Authority must be respected), the people involved didn't want to finish and be sent to some other crazy military assignment doing God knows what, most were not keen on Hitler winning the war, and a real shortage of resources because of allied bombing made progress difficult.
One of the most interesting stories he told was off a brash American officer who came into the lab after it had been captured by the allies. The office asked if there was anything of value in the facility. The scientist looked at each other and said yes the neutron sources and the plutonium. The officer asked them how valuable those things were. The scientists thought for a while and told him several millions of dollars. The officer had them all confiscated and put one in his pocket. The scientists told him they were deadly. He didn't believe them. They later learned he died of his stupidity.
I don't have the book near to hand at the moment, but I recall that it was something like that which gummed up the program and prevented any real progress. Other scientists debated whether Heisenberg had made a mistake, or had done it deliberately.
One of the most interesting stories he told was off a brash American officer who came into the lab after it had been captured by the allies. The office asked if there was anything of value in the facility. The scientist looked at each other and said yes the neutron sources and the plutonium. The officer asked them how valuable those things were. The scientists thought for a while and told him several millions of dollars. The officer had them all confiscated and put one in his pocket. The scientists told him they were deadly. He didn't believe them. They later learned he died of his stupidity.
I guess he had never heard of Madam Curie.
As it should happen, I knew one of the men tasked with rounding up and Transporting German Atomic scientists at the end of the war. He told me how he had handcuffs and a 1911 and he made it clear to them that he would shoot them if he needed to. He said they never gave him any trouble.
He passed away a few years ago. He was a very interesting man. He also went through the Iranian revolution in 1979, and he absolutely hated Jimmy Carter as a result of his incompetence in handling the whole affair.