Here is a link to a screen capture I took of my pro tools automation issue.
http://www.shatteredeye.com/protools/virus-automation.mov
Here is the problem. I set a parameter in the plug in to automate (in this case cutoff on filter 1 on channel 1). I write enable the track and I twiddle the knob. The first time in a session it WILL write this automation. Lets say I dont like it so I want to write over it. I write enable the track again and twiddle the knob. This time however it will only record the STARTING position, and nothing else. When you push stop it will record the end position, so you get a very gentle slope. If you delete the points or the automation entirely and try to write again it will do this. In the video all three times I was just randomly turning the knob on the TI.