I talk a lot about player hooks and ideas that can be used to draw them into play, such as in A Trick. It's one of the best things you can do to make a game personal. But it doesn't always work as smoothly as one might like. Players are often quite hesitant to come up with hooks for their character.
Now, there area lot of reasons why this might be so. Sometimes it's pure disinterest - as much as we rail against the game as the GM's story, sometimes that's exactly what people are looking for a chance to play in - but more often it's a result of bad experiences. A novice player tends to cheerfully create extraneous detail, but a player with moderate experience might shy away from doing so after a GM has ignored, quashed or profoundly abused the player's creativity.
In such a case, there's a temptation to try to force the player to try just one more time, but that exchange tends to be reminiscent of getting a child to eat vegetable. it might be funny if it weren't for the fact that we're all grown ups - rather than try to force the issue, step back, and consider a different approach.
If your players aren't inclined to produce hooks, then it's time to raise a discussion about a seemingly unrelated issue - group cohesion. As a GM, I spend a lot of time stressing about keeping the group together, but the reality is that this is a task that you can reasonably offload onto a responsible group. Making it explicitly clear to players that keeping the group together is their job is a useful thing to do. It not a matter of putting forward a threat, or framing ti in punitive terms, but rather one of getting player agreement that, as players, they're a team, and no one gets left behind. This may seem complicated, but if your group gets along, it's really just a matter of having the conversation in the first place.
This becomes important once you can rely on the connections between your characters. if those connections are reliable, then it only takes one or two hook laden character as their hooks become, effectively, secondary hooks for the hookless characters. This can be very liberating for those players and allow them to focus on the kind of relationships they're looking for in play.
Often these are straightforward things like friendship and camaraderie. It's very satisfying to have a friend's back or to help a buddy out in a pinch. Many players who are looking for more positive relationships than they trust the GM with can potentially find them within the group, with the "bad" GM stuff providing the fodder for reinforcing those relationships.
Now, I know you hardcore GMs are flinching about this kind of warm and fuzzy stuff, but step back for a second. You're still going to have all the opportunity in the world to throw in nasty stuff, but by putting pressure on the group in the parts that are expecting it, you draw in the other players without them feeling like they're getting yanked in directions they didn't sign up for.
This is one of those cases where a soft response can get you a lot more buy in than a hard one. Forcing players into something they're uncomfortable with "for their own good" is less likely to get the play that you all want then simply making sure the opportunity is there for them, in the form of their friends and companions.
1- If your group doesn't get along, this is probably the least of your problems, but this may at least help them find a way to get along better.