It looks like I've solved my problem. Long story short, I was jumping through some funny hoops in an attempt to define dV/dtheta (i.e. differentiate w.r.t a function) using diff, when I needed to use D (and indeed the answer I got with diff was completely incorrect). As of now series seems to be automatically doing what I want, although I'm going to do some more testing to check. Thanks to all for your help!