Several factors can create weak joints with any glue, but superglue joints may fail because:
- Too much glue makes a weak joint. The glue seems to crystalize and weaken over time, especially when too much is applied.
- Differences between materials joined - e.g. the brass details glued to white metal. Different rates of expansion and contraction due to temperature changes (even if minute) can weaken a joint over time.
- Some material is notoriously hard to glue with any glue - e.g. slippery delrin plastic usually used for trucks.
- There was not a good fit to start with, and the glue did not penetrate the material (even a few molecules' depth), rather it was strictly a surface to surface joint.
- Possibly (although not likely) there was not enough moisture to properly cure the glue. Superglue was originally developed as an alternative to stiches. It draws moisture from the skin to cure (which explains why you can glue your fingers together faster than model parts). If there was not enough "atmospheric" moisture to cure the glue, such as on a really dry winter day, it may be weak.
Andrew