Isn't it possible that in the search results web part, in the 'Performance' section is the 'Disable viewstate' property enabled (checked)? (sample screenshot) This could cause that the control does not use viewstate - which is required to change the state of the control.
If not, check the same setting for the web part zone, this time in the 'Default' section (screen shot). And if it is not on that template used by the page, check the master page template and the same setting in the page placeholder web part. (screen shot)