Because Pokemon that aren't the same species will sometimes not like each other. Also, take note that a Pokemon conceiving an egg is pure rng and it is completely unreliable even if the chance is ninety nine percent there is still that one percent that they won't conceive an egg
In your case, you just have to walk around some more until they conceive and make sure you walk two hundred fifty six steps, which is just about a max repel
However, do take note of what the lady says. If she specifically says 'They don't seem to like playing together, though' then your Pokemon are never gonna conceive an egg
Source: Experience and this https://bulbapedia.bulbagarden.net/wiki/Pokemon_breeding
