C# - Multithreading On A Multi Core Machines Not Maxing CPU?

I am working on maintaining someone else's code that is using multithreading, via two methods:

1: ThreadPool.QueueUserWorkItem(New WaitCallback(AddressOf ReadData), objUpdateItem)

2: Dim aThread As New Thread(AddressOf LoadCache) aThread.Start()

However, on a dual core machine, I am only getting 50% CPU utlilization, and on a dual core with hyperthreadin enabled machine, I am only getting 25% CPU utilization. Obviously threading is extremely complicated, but this behaviour would seem to indicate that I am not understanding some simple fundamental fact?

[code]...

And this is occurring in a recursive function, so the QueueUserWorkItem can be executing multiple times, which in turn then executes exactly one new thread via the aThread.Start

Hopefully that gives a decent idea of how things are happening. under this scenario, should this not theoretically pin both cores, rather than maxing out at 100% on one core, while the other core is essentially idle?