That is because the MIDI events in a MIDI clip are sent directly to the plug-in, not to Reaper's Automation path.
Now if the plug-in is programmed to respond to MIDI CC messages then you will see a change. Here is an example, using LinPlug's AlphaFree with CC# 01 and 74 controlling two of the Filter knobs.
This is MIDI CC automation and does not involve Reaper's plug-in parameter automation at all. You could do it in any host.
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On the other hand If you want to use MIDI events in a MIDI clip to be used by Reaper's plug-in parameter automation, then they need to be routed into the Automation path. This can be done using Jeffos' MIDItoReaControlPath
http://forum.cockos.com/showthread.php?t=43741
but that takes a bit more work to get going