When I arrived on Islay, I was introduced to a lot of people who told me they'd forgotten all their Gaelic. But by the time I left the island, several of these people would regularly speak to me almost exclusively in Gaelic.
It's tempting to call these sorts of people lazy (and I've heard many people do so), but I think there's a wee bit more to it than that.
They genuinely, genuinely believed that they couldn't speak Gaelic, because they genuinely found it difficult. Why is it difficult with other people but not with me? Because they learned to expect Gaelic from me, but they had learned to expect English from other peopled.
This expectation can be overcome, but it's a hell of an effort, and most just don't have the motivation to do it -- in fact, most people don't even know that that's what's going on. It's not their fault.
So how do you encourage people to overcome this inertia?