I have a Photo-blog and long time back I had created a a brush to watermark my photographs before I put them online. Few days back that brush gone away from Adobe Photoshop Elements. I was wondering the reason for the same and the best guess I could make was - custom-brushes are swiped on loading new burshes which are downloaded from free online websites.
So best way to keep custom-brushes is to save them after creation. In Brush selection dialog, there is an option to shape any of the brush. So whenever you create a new custom-brush, please save it explicitly. So that in future, it can be reloaded again after some problems with custom-brushes in Adobe Photoshop Elements.