Baseparts that are descendants of a basepart/model parented to a persistent model will never load on the client. This occurs only if the persistent model is outside the streaming area (it would be streamed out if not persistent).
Conditions:
A persistent model exists in the workspace.
The player is outside the streaming area of that persistent model.
A basepart/model containing other baseparts is parented to the persistent model.
Result:
Descendants of the basepart/model that are exclusively baseparts will not show up on the client.