According to the help window
It's possible that what happened is another power prepped more to a system that was too close and caused the other to fail to go into expansion. There does seem to be a radius.
I had several threads in the beta forum charting how, over successive cycles of the beta, the top ten prepared systems for Aisling Duval didn't match up with the systems actually selected for expansion.
It wasn't a case of another Power prepping more there, it was just that some systems were passed over.
FD claimed to have fixed this in one of the latter beta releases, but we never got another cycle completing before the beta ended to allow me to confirm.
I wouldn't be surprised if the 'fix' didn't, as other items listed as fixed throughout the beta frequently weren't.
As to the nomination mechanic, all the nominations do (or did in beta) was provide extra preparation for a system without having to prepare it manually.
So 10 nomination points was just equivalent to carrying 10 units of powerplay cargo to perform a preparation.
Initially you would add this at the preparation system similarly to providing powerplay cargo, then later in the beta they added a button on the PowerPlay preparation screen to add to systems in the top ten.
If there isn't any systems in the top ten, you need to manually prepare systems until they appear in the top ten list (which might require getting the 100 preparation points, not sure).