Why navies struggled to manufacture very thick armour plate

Did European navies also struggle to make armour plate as thick as designers wanted?

Yes, because all foundries faced limits in producing and heat-treating very thick plate consistently.

The problem was not unique to the United States. European manufacturers also advanced in stages and experimented with overlapping or composite arrangements when one plate could not provide the desired performance. At extreme thickness the centre might not set correctly, producing uncertain protection despite greater weight. The exchange shows that nominal armour depth cannot be separated from metallurgy and manufacturing quality.