Hi David,
the first time I did it I remember I experienced something similar, because of some mistake I did.
Could you explain us, maybe with a couple of screenshots, how you're trying to do that?
I cannot make screenshots right now (I'm away from my workhorse computer); open the directory managar; click on the name of the library for which you want to change the preload buffer size, the string with the name of the library and the folder's path should became highlighted. Now change the buffer size.
At this point you should see the different preload buffer size near the highlighted string. There's no "save" or "apply" to click. It has to work!
Hope it helps...
Fabio