[S/M] How to teach Chansey Seismic Toss

[Dionysus]

Good Evening everyone (at least it is evening here)

I want a Chansey knows Seismic Toss however, even I followed the guides, bred my Blissey with a Mawile(knows Seismic Toss) the off-spring Happinys do not know Seismic Toss and after I evolve them still not leart.

Is there another step I should follow during breeding process?

Thank you.