I think it's a natural feeling. But it's probably a sign that you're doing too much active facilitation at the moment. I realize they're asking for the projects, and you're doing the good-unschooling-mom thing by responding and facilitating. But if you're left feeling you've done most of the work and they don't really care about following through, it's probably a sign that you're facilitating too actively for what they really need just now. Perhaps your active facilitation has turned it into something that in their minds is no longer about their interests but more about you. Even though yes, they asked you to help, now it feels like you own the endeavor, and that has taken the wind out of their sails. Or perhaps they didn't really want to do whatever it was much in the first place: it just sounded kind of neat so they spoke it out loud but really if they'd thought it through they'd have realized they didn't really want to put any energy into it. Or maybe they were enthusiastic, but it took time to procure materials and come up with a plan, and they've moved on: the interest was just a flash in the pan.
I know I was sometimes guilty of fanning a spark of interest too vigorously ... and extinguishing it. When kids are not appreciating the fanning I think it's helpful to just back off and leave a bit more up to them. The longer they want something, the harder they have to work for it, the more they'll be invested in it and more effort they'll put into seeing it through. The worry for the parent, of course, is of missed opportunities. What if that interest in seed germination passes because I didn't jump on it, and a valuable learning opportunity is lost forever? Am I being lazy by not responding, or putting my kid off? And it's true, there will be lots of little interests expressed in passing that will just fizzle out. But really -- that happens, as you're seeing, even when you do jump on interests. And some won't fizzle just because they experience a little benign parental neglect. If the child has to put a little intent and persistence into making them happen, she's far more likely to be invested in her learning.
So if your kids say "We want to grow a herb garden" or something, you might consider saying "Oh, okay. Sounds cool." And just leaving it at that. Two days later you might hear "Mom, I said we wanted to grow herbs!" with a whiny help-me kind of overtone. And then you might ask some guiding questions, but still not step in to plan and organize: "On the kitchen windowsill? Or outside? What kind of herbs do you want to grow, anyway? Have you figured out what you want to use for pots?" Just gently volley the ball back into their court.
So unless the project holds intrinsic enjoyment for me, I try to make sure there is some persistent organizational energy coming from my child, and it's not mostly falling to me to prepare and organize. If the project does interest me, I sometimes choose to facilitate it with eagerness -- but only if it's something I know I'll enjoy finishing up on my own. For instance, right now I'm in the middle of a bunch of shibori dyeing. Younger dd had been introduced to shibori in an art class and was interested in doing more of it. I loved the idea and suggested doing a quilt with shibori sampler squares. She was eager, did a bit, then lost interest. But I'm really happy finishing up without her.
So yeah. Less active organization, more of a low-key facilitation. Or if you do actively organize, make sure you've made your peace with the possibility of finishing it all up yourself.