If I had to pinpoint a moment in any given story where I become truly invested in any character, it's the moment where that character is faced with a difficult situation and they are forced to let it define and defeat them or they finally summon what's inside of them and fight back like a kickass ninja. After all, in our own lives, these are the moments of greatest growth, where we show the world what we're made of and who we've become; how far we're willing to be pushed and what we're willing to accept before we say "No more!"
If your character(s) in your story never have a moment like this, that's a great place to start as you deconstruct what might not be working as well as it could. Your novel does not need to be a high concept adventure or fantasy novel for your hero to have a defining moment. This can also happen in a contemporary romance when someone has to reveal what's in their heart, or in a drama when someone is forced to face their fears and work through them to overcome an obstacle, or even in a comedy when the unlikely hero has to take a stand, or does something he never dreamed he could do that changes everything.
Life is filled with things, people and circumstances that we cannot control. Our want to is what makes us human. Or success or failure at it shapes who we are. It is critical to include these elements into our stories to make them relatable and realistic. Sadly, in most situations in real life, the puzzle pieces do not all fit together easily, all the threads are neatly tied together and everyone lives happily ever after. We can take every measure possible to block out the darkness and the pain and the things that unsettle us, but when they find their way in the cracks, that is where the drama lies that has the power to rattle us, to bring us to our knees and to challenge our spirit. To create a truly memorable and meaningful character, his/her life should hold no less in whatever manner feels appropriate for your story.
And just as in real life, sometimes we are forced to accept what we cannot change and to know our limits. Sometimes that brings about new beginnings. Don't always give your characters the resolution one might expect. Good luck!! Now get busy writing!!