Reasons for remaining drv violations: Could not be fixed because there is no usable buffer or delay cell for buffering

After a optDesign -postcts, the tool reports:

Reasons for remaining drv violations:

Could not be fixed because there is no usable buffer or delay cell for buffering.

I'm sure that library have buffer because they have been added during CTS stage.

So my question is: why optDesign can't find buffer for drv optimizations ?

Using INNOVUS 17.1

thanks