When you look at the place a scene needs to go and go blank, suspect this. I don’t mean a weary turning away from boredom: I mean where part of your brain is admitting “I don’t know how to do this. I want the scene to do these things, but I don’t know how to make it happen.”
Everything has a learning curve. Just because you can mix colours doesn’t mean you can paint a real likeness in a portrait or make a landscape not only convincing but dramatic. Equally, just because you can write sentences and paragraphs and have a good vocabulary doesn’t mean you know how to make a scene work dramatically or make a dialog zing.
Alas, the only way to learn to write a scene is to write it. If you don’t like what you get, keep the parts that work (not necessarily the cleverest bits) and rewrite it. Over and over. Trying again in a new way is what all creative artists do to get it right, to approach closer to their vision for the work.
It may be you have bitten off more than you can chew. Beginners in writing, for some reason, frequently refuse to admit they are beginners. They expect to spring full-grown from the head of Literature, an instant genius and respected artist. No choreographer would expect to go right from college to a major company. No one learning their chords expects to sell a musical to Broadway next month. But for some reason many freshman writers get all tripped up by their ambition and ego. They are imagining the brilliant book they want to have written, all the praise written about them in reviews, but they really don’t know what goes into it. They usually have excessively complex plans that show off their genius by refusing to be limited to what everyone else does, like cohesion and comprehensibility.
It may be a genuinely brilliant idea but, as such, it’s not something a beginner or maybe even an intermediate can write. You may have to keep it on the back burner until your skill reaches the level required.
The answer to this is rarely to get a collaborator. Most writers are terribly busy with their own ideas. All those collaborated books you see (mainly in fantasy) consist of ideas and outlines and some world-building from the Big Name’s notebooks, with most of the actual labor done by the newer writer (but they split the take 50/50). Some are arranged over coffee, but most of these are brokered through the publishing house, which would like to exploit the Big Name’s audience more and also build up that audience for accepting the New Name. Sometimes friends do get together on this. Maybe one person has a full or partial manuscript, and their beta reader sees where it could be built up to new levels. (Fraterfamilias matured this way.)
The real answer is to write simpler challenges until you are up to this.
For those of you who want an immediate fix rather than enduring three years of consistent work on other projects, I will suggest a painter’s method. Painters of the old school taught themselves solutions by copying things they wanted to be able to paint, so that they honed their observation of details like the filtered light that makes the thin flesh of a nostril or ear glow a bit.
For a writer, try to find scenes with the same problems as your scene, and see all the different ways other authors have handled the same difficult bit. I will even suggest you key them in or copy them by hand, word by word. It makes you pay real attention rather than just skimming.
On the other tentacle, you may need a partway revision to see if the problem with this scene is actually that it shouldn’t be here. Maybe you can’t make it work because it’s not really what the characters would do, now that you know them better. Your Muse is holding you back rather than violating the characters with Author Motivation, you lucky soul. In your heart, you know it’s wrong.