Derivating problems

[wheazel]

Hello Community,
well, I am running into the same problem, and after research on the net, I found an old thread here in the forum that looked promising...but I ran into trouble and ended up in this thread.
What I've been looking for is to get the second derivative of a function. Sadly, I can't get it done.
As I understand, the only way to do this on the Prime is, to enter:
diff(f(x),$(x,2)) or, what seems the same: diff(f(x),x$2). Both Versions return a 0, which is not correct.
diff(f(x),x) works fine, so does f', f'' and so on.
I made sure, that I'm in CAS mode, and that the variables x is not in use.
btw..
f(x):=1/x^5

Thank's in advance for any suggestions, corrections or help