UPDATE: So I called this "foolproof," but really, it ought to be called "fool-resistant," because even the most bulletproof technique fails now and again. Some people have reported that the mayonnaise never comes together when blending. The number one problem I've discovered is using the wrong jar. It is imperative that the jar be just slightly larger than the head of the immersion blender, as the egg/lemon mixture must be in contact with the blades of the blender before you switch it on for this to work. The head of the blender must be firmly planted against the bottom of the jar until the mayonnaise starts to come together. If you can't find a jar the right size, the other option is to double the recipe in order to increase the starting volume of the egg/lemon mixture.

Finally, if your mayonnaise is watery, that means it has not emulsified properly, and no amount of additional blending is going to fix that. Your best bet is to let the mixture settle and separate, then try again.