Your last sentence. Quite possibly
It's rare, but I've had a few times feeling the same way with a few of my cats. Then I relax and remind myself that they are probably not feeling the same as I am. That's pretty evident in the way they react to each other.
Sometimes, they will withdraw from you, I know it seems like you feel responsible for them withdrawing. The good news, is that it is usually only temporary. And then you will see new behaviors, new relationships forming, new activities, new everything, it's sometimes a change for you too. They are growing and you're not willing to give up the one-on-one you had.
But, you don't have to.
Certainly, Chunk needs a great deal of your attention, and you do have to accomodate his kitten needs. But, at the same time, you don't have to change too much, just divert his attention to other things and do what you can to make Willow feel important as always. Try not to change routines, or those times you and Willow are accustomed to being together, just increase your time with her, and try to engage them both in similar activities, playing together, etc.
Buy new toys for Chunk. The novelty will grow on him and he might start playing more with HIS things rather than Willow's. Create a special area just for him. You can't change the fact that he wants to fit in, with Willow, with you, with the family, and he will, unintentionally, move right in. This is something Willow will, in time, learn to tolerate and accept. Just make all other things pleasurable for Willow as you can.
Part of having a multi-cat household is the expectation the cats will, and must, share everything. It may not be their desire at first, but they will manage, and they will adjust.
Remember, part of what you think is "withdrawal" isn't exactly that. Willow is adjusting to someone new, and is taking her own sweet time, it doesn't mean she's mad at you, it is probably just her way in this adjustment period. Do, however, encourage her every time you can, to continue your bond. When Chunk is napping for example, do something extra special for or with Willow. Just take a nap with her, sometimes that's all it takes for her to feel secure. Sit with her when she eats. Sit by her when she's watching out the window. Just your calm presence will remind her she's still your baby.
Try a second food station if you have room. Or two dishes. Rather than teach Willow to use the second station, teach Chunk that the second station is his, for now. Literally and physically place him at his station as often as you can so he makes the association. You can't really stop his curiosity between food dishes, all cats do this, as they seem to think what is hers is mine, etc, and they seem to think one of the dishes has something better (even if it doesn't, lol).
Remember, Chunk needs all the love and support you can give too. He's so young, and growing and developing, and I personally think Willow is a good match for him. She may not think too highly of him right now, but his youth will be good for her to teach him how to be a cat, to teach him that she is alpha, and to teach him HER boundaries (with a little help from mom - like the food dish or the tunnel, sleeping spot, etc.). If you have two spare blankets or throws, place them in the intended spots for sleeping areas. If Chunk wants to get on the bed, teach him that his throw is a foot or so away from Willow's. (not saying this will work, but it's something you could try). Sunshine on the bed is a favorite for ALL cats, so try to accomodate as best you can, or put a kitty bed by a sunny window.
Give it time, and be patient, and of course, loving and pampering to both of them. Don't let your emotions be seen or felt by Willow, try to act like everything is peaceful and in harmony.