Resource Levelling in P6V7 - Some activities cannot be levelled

Hello P6 Community,

I am levelling my test schedule activities by selecting only the over-allocated resources. After levelling, in the "Resource Usage Profiles" I see that some resources do not show any Improvement, though, all activities are "Resource Dependent". The Schedule Log tells that there are a large number of activities that cannot be levelled. Primavera user manual and help do not explain this nor can I see any clear pattern of parameters among the non-levelled activities. I wonder if anyone from the Community has any clue.

Appreciate your thoughts.

User298873