Many instructions in bee-keeping start with first find your queen. This can be far more difficult than it sounds, even when the bloody queen is marked with a huge blob on her back.
To determining if a scene is flat, you need to first find your turning point and more important to make sure that turning is active rather than passive. Like the beekeeping instruction of first find your queen, it is more difficult in practice than it sounds.
So here is a small formula:
1. Determine the POV character. (if you don't know whose POV you are in STOP. You will have a flat scene as the emotional connection is not there.)
2. Determine how the POV character is different at the start of the scene v the end. This can be a massive change or it can be an understated change. (if no change.STOP! You have a flat scene or you are using the wrong POV character)
3. Where and when does this change take place? (this is the turning point of the scene. It is the pivot on which the entire scene turns. Is this the right change for your character? Should there be a more definate change? Is the change too small or too big?)
4. Is it on the page or in your head? (if in your head, STOP! You have a flat scene as the reader can only know what is on the page. Readers can't see what is in your head. This is often a major failing of my early drafts.)
5. Who ultimately causes the change? (if the changes happens to the POV character. STOP. you have a passive turning point and possibly a reactive character. The most interesting characters are proactive and make things happen even if they make mistakes. Note if it is caused by the other POV character -- most series romance is dual POV, you might be fine as this is a special case. However you do need to be cautious that all changes are not caused by the same person)
Once you know where the turning point is, who causes, and what the change actually is, you can go about highlighting it and bringing it out more in manuscript. Either by higlighting the change. SHOWING the difference or heightening that moment of change. If a scene shows a definite change and the change fits in with the growth arc of the character, then the scene is not flat and it probably has earnt its place.
Scenes need to move the story forward in some way. They can only do that if the POV character changes in some fashion. Even scenes which essentially exist to impart info are about change. How was the character feeling before they had this piece of news v how were they feeling afterwards?
If nothing changes and the scene is merely recounting, see if you can cut it. Or alternatively see if you can heighten the feeling of change.
As with everything in writing, it is much easier to say than do. But if it was simple, writing wouldn't be as much fun!