Answering this needs some context. Like quite a few German technological projects of the WWII era, the German nuclear project was ill-founded and under-resourced. The importance of heavy water was that it could be used as a moderator for a nuclear reactor, which could be used to make plutonium.
The German project was well aware of these things, but had failed to appreciate the difference between the fast-neutron reactions that you need for an atomic bomb and the slow-neutron reactions of a reactor. All of this is quite clear from the Farm Hall transcripts published by Jeremy Bernstein in Hitler's Uranium Club.
The various members of the project had tried to get resources, but had been self-sabotaging. Senior academics had great prestige in inter-war Germany, and the project members had not been obliged to learn how to explain the basics of their knowledge to people less knowledgeable than their doctoral students. Nor were they good at admitting to themselves, or each other, what they didn't know. So they turned down offers of large resources, saying that they knowledge to make use of them was not yet available. Two years later, it was clearly pointless to ask for huge resources, so they didn't.
By contrast, the Manhattan Project had been set up by people who know how to communicate with academics, and its scientific leader, Oppenheimer, had been chosen for his unusual gifts in getting physicists to work together. It also had Enrico Fermi, who was gifted in both theoretical and experimental physics, and was one of the best engineers ever to win a Nobel Prize.
One outcome of all this was that as of August 1945, when the German physicists detained at Farm Hall learned of the bombing of Hiroshima, Werner Heisenberg, the head of the German project, while aware of the concept of a critical mass, had no real idea of its size, and sat around talking about it, rather than attempting to calculate it. He was a quantum mechanics theoretician, and a gifted one, but not a practical man or an organiser of teams.
The German Armaments minister, Speer, does refer to the German nuclear project in Inside the Third Reich, saying that he'd given up on it by mid-1943, when he released the stocks of uranium metal to be used in anti-tank ammunition. The physicists had a strong personal motive to keep it going on a small scale, in that it was work they were interested in and relatively safe. Getting conscripted into the army and sent to the Eastern Front was always a prospect for them, and was close to a death sentence.
Their post-war claims that they deliberately withheld atomic bombs from Hitler don't stand up, although they were doubtless good for their self-esteem. They didn't do the right things at the start, and never had the resources to catch up.