Sometimes a scene doesn't work, but you can't put your finger on why. There's conflict, there's a goal, a protagonist and an antagonist, a climax... And still it won't come to life.
The best way I know to tackle this problem is the following, which was taught in my postgraduate screenwriting course by a British script-editor/writer/teacher (and now also drama therapist) called Charlie Moritz. You do need at least one partner for this technique, which makes it perfect for classroom/workshop situations, or when you're working with a regular writing partner.
Basically, the technique is this: you act out the scene, but you let go of the written dialogue - and you take care to notice your natural reactions/impulses in the situation. Do you turn away at a certain point? Does it feel unnatural to skip to a different subject? Do you want to get closer to the other character in the scene, or conversely try to escape their presence? Body language is extremely important here. Often you will physically realize that a certain gesture or action described in the script just isn't right. In this way, you will instinctively be able to determine where the scene starts to go off the rails.
Once you've done this, you can now try to integrate your natural reaction to the events within the scene into the rewrite. Sometimes this will be very easy - a simple refocussing, adding or deleting a scene beat, or taking more time to register a certain (re)action may be enough.
But in other cases, you may be faced with the fact that your natural impulsive reaction totally contradicts what the scene is supposed to convey. And if you're really unlucky, this may have serious repercussions for the script in its entirety.
Nevertheless, the natural reaction is always truthful, and that is extremely important in screenwriting. So paying attention to this truth is highly recommended.
Now, some people may object that this is all well and good, but all you've done is discover your own impulsive reactions. And the character in the scene is a totally different and separate character from the writer!
That's why it's important to translate what you've discovered about your own truth to the character and their situation. Combining your natural reaction with the psychological make-up of your character can lead to discovering a different, perhaps surprising way to get the same story point across.
In any case, you will now be able to rewrite your scene starting from truthful human behaviour, rather than from 'unreal' imaginings which have no discernible basis in psychological reality.