Hi Nanny Jan
Sorry I can't really be of any further help. The indexes do not say why this was not completed. I have checked some original bindings I have for my family and it is unusual to mention why the apprenticeship was not completed. I have checked the reassignments and nothing is listed for Henry Langham.
A normal apprenticeship lasted 7 years and therefore whatever caused Henry to stop his apprenticeship must have happened before 1860, are there any clues in the 1861 census?
Regards
Jamie