Sometimes in Pro Tools you might notice that you aren’t able to write plugin automation even though your track is in the appropriate auto mode.
There are two things to check if this is happening.
From the plugin, open the Plug-In Automation Window by clicking the button directly beneath “Auto” towards the top of the window, and be sure any parameters you wish to automate are added to the column on the right side.
In the global Automation Window (Command-4), be sure that Plugin-In is turned on under Write Enable.
In general, be sure that the type of parameter you want to automate is enabled under Write Enable in the Automation window.