Over time oils can gum and go varnish like. This means they are no longer lubricating moving parts such as sears and single trigger mechanisms. The latter, depending on mechanism have quite a few small moving parts weighted by little springs. Sears also use little springs. Any gumming of these parts can and will cause malfunctions.
Mostly these can be easily sorted with a good clean and re-oiling with a light non gumming machine oil. Over time little springs can and do corrode and thus break.
Boxlocks and really simple, whereas sidelocks are much more refined. What is needed is a knowledgeable gunsmith who can take basic parts, either from raw metal, or part finished, shape them to fit, harden and temper and then do the final fitting. The hardening and tempering is where the real skill and knowledge is.
Most side by sides, including AyA are assembled by hand by craftsmen taking rough parts, finishing and then fitting them. This pretty much means that parts are not interchangeable between one gun and another. But they fit well. And its why AyA sidelocks now start at well over £10,000 plus.
Its also why they need proper servicing and its not a five minute job.