Some people find that the Spotlight search suggestions feature of iOS 9 simply doesn’t work. Activated by swiping right from the first home screen, Spotlight search should let users quickly search through emails, music, videos and also let users search certain apps. But those affected see nothing for any search term they type.
It turns out this is a problem with devices that have 1 rather than 2GB of memory, and is caused by the the Spotlight index file getting too big. iPhones before the 6S/6S+ and all iPads earlier than the Air 2 have only 1GB of RAM.
The solution? Perhaps the quickest and easiest thing to try is to restart the device. In fact, some experts advise restarting every iOS device periodically — say once a week — just to keep it healthy, in any event. To restart the phone, press and hold the sleep button, then drag the Slide To Power Off button. When the phone has shut down, press and hold the sleep button to turn the device back on again.
Alternatively, you can try using an app like Battery Doctor, which includes a routine to allegedly free-up memory. Just tap the Memory button, then the Scan button, and follow the instructions.
If either of these solutions don’t work, and as a last resort, you can try the ol’ wipe and restore trick, although beware that this will wipe your Apple Pay cards and also your TouchID details, requiring them to be setup from scratch upon restoration of iOS.