Are you putting them in a community tank? I have a pair in mine, they have took possession of a coconut cave I put in, I also added a piece of slate inside it too.
They are definitely spawning, the female is in and out all the time and the male stands guard at the entrance, any fish come near he chases it away, she also chases other fish away if she is out too.
I have 2 more Kribs but they are not paired up and they too are chased away, they chase fish twice their size, very protective but fun to watch.