This issue doen't seem to be very adaquately explained. There could be many reasons your Reshiram and Kyurem are not fusing, but your answer could be:
A.) You already have fused set of Pokémon. Any fused Kyurem needs to be unfused before the DNA Splicers can be used to fuse again.
B.) You're not clicking on the right Pokémon. Make sure you click on Kyurem and Resiram.
C.) You could be clicking on the Pokémon in the wrong order. Try tapping Kyurem first, then hitting Reshiram.
D.) Albeit highly unlikely, it could just be an awkward bug in the game.
I hope I helped.