NetworkIdentity objects not always being spawned on client?

In the game that I'm working on, items with network identity are often spawned during the middle of the game using NetworkServer.Spawn.

My users have been reporting to me that when a game goes on for awhile and a bunch of items get spawned, eventually the items just stop appearing on the client side (they do appear on the server). The rest of the game works fine. The issue apparently fixes itself when the server changes the scene. I haven't been able to reproduce the issue myself, but I've gotten a number of reports about it. I should note that I have a lot of NetworkIdentity objects in my game at any given time (100-150+).

Any ideas why this might be occurring?