[ThRoewer]

One of the things that worked well for me to induce spawning with clownfish pairs is adding pairs of dottybacks (P. fridmani is ideal), damsels or Marine Bettas. Especially the dottybacks and Marine Bettas spawn quickly if you make pairs of sexed adults. I had Marine Bettas spawn 2 weeks after pairing them. Dottybacks generally get going after a month or two.
My theory is that the released hormones from the other pairs get the clownfish going.
Of course you could also add the hormones directly (should be available from suppliers of fish farm equipment, foods and medications). It's a pretty common method in the commercial fish farming business (the ones breeding food fish).