I am using Creo Parametric 7.0 220.127.116.11
While adding components in the family table, some parts are not getting selected in the assembly.
Going on the information that you have given:
If the parts that are missing have constraints tied to parts that are being suppressed, they too will be suppressed even if you have them listed as active in the assembly. This is a possible place to start.
Are you refreshing your workspace to bring in the latest generic with the added instances?
Are you revising all instances when adding new ones? I have found that this helps and prevents multiple iterations of the generic being utilized.