Currently I am trying to make a mask around text so I can reveal text to make it look as if it is being written by hand. I place the text down with the text tool, I then used the pen tool to trace the text. For some reason though, I can only place a few anchor points before a bounding box appears around the anchor points. I can no longer add anchor points to my current path.
If I use the pen tool to select my created path, I still cannot add more anchor paths. If I restart after effects I can sometimes place a few anchor points on the path then the bounding box reappears and I cannot place anymore.
Do it on a solid: not the text. Change the solid's opacity to about 50% so you can see what you're doing. When done, change it back to 100% and use Write-On or Stroke.
I usually set a solid to a vibrant color, change the blend mode to screen or multiply so I can see through it, and create and animate may masks on this solid. The solid is then used as a track matte. It lets me accurately see what I am doing.
If you close your path or miss a click when drawing a path you will occasionally get a bounding box. Pressing escape or deselecting the layer, then carefully selecting the path again should let you continue. It just takes a little practice to be able to add points to a mask.
BTW, if your mask is complex and has a lot of vertices then you should make sure that you have all of the points in place before you start animating mask position. You'll get better interpolation between keyframes that way. Here's an example:
I"m using both the marquis selection and transform option as well as animating single points here to do a simple roto job.