Yup, I have this problem sometimes. A shiny new idea shows off and struts around in my head, but when I think about actually starting to write it, something isn't right.
I've learned that this means that I don't have a fully-formed place to start yet. The idea needs more development, or it simply isn't enough on its own - it needs two or more friends to jump into bed with before something awesome will spawn and want to be written.
The most recent example of this for me was Tales from the Screw Loose, a project that has been on my mental back-burner for a couple of years now. The initial idea for a robot brothel was heaps of fun to play with - it had characters and a setting right off the bat, not to mention so much hilarity - but there just wasn't enough there to start writing. So I rolled it into a back corner of my brain and let it fester. Much later, I came up with an interesting idea for one of the colonies in the Starwalker universe, and Screw Loose piped up from the corner, waving its little hands for attention. Suddenly, it had a much bigger setting, ties into my current project, and a whole bunch of new material to play with. It expanded, deepened, but still wasn't making me itch to get started; it was still a fun situation but lacked a real purpose as a story (or, as I like to call it, plot). So I sat down and wrote all the material and ideas I had for it out on notecards, played around with it a bit, played the 'what if' game, let it percolate for a while, and finally a plot rose into view. Now it's roughly ready to go (all I need is the time to start writing it).
It took me a while to figure out that an idea that seems great but doesn't want to be written yet is not complete (for me, at least!). It has taken longer for me to be able to articulate what's missing when that happens. A lot of it is simply analysing and learning my own writing process, and the way that I combine and develop ideas. Most of the time, it just needs time. Other times, writing it out can tease out more of what I need, or brainstorming, or imagining how it would go if I wrote it as a comedy or horror, or bouncing ideas off a friend. As Wildbow suggested, playing with alternatives within the idea itself can work, too. Smoosh it together with another idea that doesn't quite work yet, adapt and keep smooshing.