Ableton Live by default doesn’t pass through all keystrokes to active plugin windows. There are flags that can be set in Options.txt, a text file you can create in the Live preferences folder.
**EnsureKeyMessagesForPlugins (Windows only)**
Line to add to Options.txt file: -_EnsureKeyMessagesForPlugins
This option solves a problem with certain VST plug-ins (such as NI's Reaktor), where keyboard data does not reach the plug-in.
See this here: