Does SABnzbd give any errors as to why it's only working sometimes? What pool setup are you using (mirrors, RAIDZ, etc...)? If you SSH into the actual OS and run top, does usage top out when trying to create the jail?
Maybe you'd be better off installing everything Usenet related in one jail like I did, it eases the installation and configuration by many magnitudes. It annoyed me that it installed each plugin to it's own jail and didn't give you the option to install a certain plugin to an existing jail, I would end up having a tangled mess of data shares between jails, problems with post processing and communicating between the jails, etc... where in the end I just created a vanilla FreeBSD jail and installed everything manually. Yea, it's not as easy as clicking a button but at least you know what's going on in the background and have control over it so you're not sitting there looking at the progress bar for (tens of) minutes thinking "is it doing anything??". Also it makes upgrading everything to a new version a breeze since you can do it from the CLI instead of clicking the update button in the GUI and wondering if it's doing anything and how long it's going to take.
