In the 'Find Similar Objects' dialog box, do you have the 'select matching' option checked? You might have only the 'mask matching' option checked which will give the appearance that all the similar objects are selected, when they actually are not. I typically leave the zoom matching, select matching, clear existing, and mask matching options checked.
You can also verify how many objects are selected in the SCH inspector after you run the find similar objects query. At the bottom of the SCH inspector, it will display the number of objects selected, and the number of documents that objects are selected on. If it only shows one object selected at the bottom, then go back and verify your Find Similar Objects settings.