Complex Objects Partially Expiring?

Hi, I'm new to JCS dev and wanted to know if anyone else had  
experienced this problem:  we've got a cluster of production servers  
running JCS in LRUMemory mode, which we're using to store fairly  
dense, heavily nested complex objects.  A strange thing is happening  
whereby sometimes I will pull an object out of the cache, and one of  
its child collections is empty, and I was wondering if perhaps JCS was  
dropping this data due to expiration or space management policies?  It  
seems strange that only part of a cached object would be released, but  
I want to be certain one way or another (there are other possibilities  
I'm looking into too).

Any feedback is appreciated!

Thanks,
Zac