Why aren't my spin savers giving me free spins?
The Spin Saver boost is not designed to give an extra spin in every case. It is only used if there is just one unused gem left during a spin.
For example, if you spin and match 2 gems, but cannot match the remaining 3 gems, the Spin Saver will not be used.
That said, if you have 5 gems to start, and match 4 of them, the Spin Saver will take away the last gem, and help you get a free spin.