I may have had a very uncommon structure, but here's what worked for me...
First of all, here's the directory structure:
--Parent
----folder1
------subfolder1
------svnexternalfolder
----svnexternalfolder
As you can see, I had my svn external folder in two different levels of the project structure, but the Jenkins project was pointing directly at "folder1".
When first configured, it would never pull the files for my svn external folder (whether it was a full checkout, or svn update). This was configured with the svnexternals at the parent level.
My next try was to remove the svn externals at the parent, then specify just the higher location on the parent, then the lower location on folder1. This gave an error since the child svn directory had the same name as the other one.
So I flip-flopped the order of creating the svn external locations and did the child first (on "folder1"), then did the higher one on parent. Once I did that, everything started working.
Hope this helps someone else.
If you're curious about why I configured the directory structure this way, this was a PhoneGap project. apparently cordova/phonegap projects create their directory structures like this, the common folder beneath the parent is the "www" which houses all html, javascript, etc files, then those are also used under the platforms/ios, or platforms/android folders (in my example, I just called it folder1).