Some wireless Keyboards don’t work with the Raspberry Pi.
Most likely due to some proprietary software drivers that the RasPi simply doesn’t like.
Keep in mind the software that is run on the Raspberry Pi is what holds the drivers, but most software for the RasbPi load up default driver module.
After reading through forums and reviews here are the best wireless keyboards I have found that are compatible with a Raspberry Pi, 2015. With links to Amazon to read the reviews.
iPazzPort Wireless Mini Keyboard with Touchpad for Android TV Box and Raspberry Pi 3 and HTPC KP-810-21S Black
This is a small keyboard with a built in touch pad that is built for a media centers such as Kodi.
Rii I8 Mini 2.4Ghz Wireless Touchpad Keyboard With Mouse For Pc, Pad, Xbox 360, Ps3, Google Android Tv Box, Htpc, Iptv (Black)
Another popular mini keyboard with built in touch pad.
Logitech Wireless Touch Keyboard K400 with Built-In Multi-Touch Touchpad, Black
Not as small as a mini but not as large as a standard keyboard is the Logitech Wireless Touch Keyboard K400.