Loading an asset bundle is null on second load

Hi, I use the following code to load an asset bundle from disk: AssetBundle.CreateFromFile(path); This works first time I run this code, but if I try to load the same asset bundle later (while the instance of unity is running), the object is just NULL. I think this is because unity has maybe cached this in memory - if so, how do I access it second time round? Thanks in advance for any pointers!