How can the Mechanicum make parts for Gloriana-class ships but no longer build additional hulls?
Short answerReplacement components require less industrial infrastructure than an entire ship, while the setting's largest construction docks may have been lost or damaged.
NotesThe fictional problem is one of infrastructure. A workshop can preserve the ability to make many replacement parts even after the facilities needed to assemble a complete giant hull disappear. The destruction around Terra during the Horus Heresy offers a plausible reason that major fleet infrastructure was lost. Rebuilding such capacity would be a long project competing with other priorities, which illustrates the real distinction between maintenance capability and full-rate ship production.