MRP runs for a site, but the component whose parents/grandparents have demands does not plan (i.e. IPAP screen is blank). Why?

Check the component’s parent part. If it has demands, but the component’s IPAP screen is blank, chances are the parent part’s Recipe or Product structure screen has an entry in the Structure Lot size tab pointing to an alternate (which is not where this component is not included. 

If you remove this entry from Structure Lot size for the parent, MRP will plan the component correctly. Other option is to make the component part of the non-* alternate of the parent (if was supposed to be there in the first place).