A few sailplanes are very difficult, if not impossible, to spin

Spin (flight)

In aviation, a spin is an aggravated stall resulting in autorotation about the spin axis wherein the aircraft follows a corkscrew downward path. Spins can be entered intentionally or unintentionally, from any flight attitude and from practically any airspeed—all that is required is sufficient yaw...

 under normal conditions.
To make these sailplanes spin easily for training purposes or demonstrations a spin kit is available from the manufacturer.

It consists of ballast weights (usually discs) applied to the tail to move the centre of gravity rearward. This increases the instability of the glider enabling it to spin.

Here is a video clip of the spin training exercise:
http://youtube.com/watch?v=XhvMJ_gs-Qk