The most common problem I see in scripts from new writers is lack of structure. I've been there too many a time myself, but I now get praised by producers for my structure, as many of the scripts they receive aren't. I discovered a little trick that helped solve that problem years ago.
When I start plotting a new script I always use the forty key scenes rule to help me, or a variation thereof if I'm writing a TV pilot episode. I don't know where I first picked up this idea, or which book I read it in, but it's been an invaluable tool over the years and really helps to focus the story telling. How do these forty key scenes work? It's really easy, let me show you.
Take forty blank index cards and stick them to a wall, or pin them to a cork board, in four rows of ten. Your first row is your fist act with the last card being your act one turning point. The next ten are the first half of act two up to the midway point. The next ten are the second half of act two with the last card being the turning point into act three. Your final ten cards are act three.
Use these cards to write down a brief outline of each scene, paying close attention to the important places as the first and second act turning points and the midpoint. You'll see if your plot has a problem, or doesn't have legs, as there will be blank spaces. It's your job to solve those plot holes and fill those blank cards. You don't have to follow this idea ridgedly, this is only a guide to help you think about your plot and work through any problems it has. You can make it as flexible or as ridged as you want. Whatever works for you.
I've adapted the forty key scenes rule a little bit since I first used it years ago and I've now incorperated a very good idea I came across in Blake Snyder's - Save The Cat! At the bottom of the cards he adds a +/-, or a -/+, used to indicate the emotional change in the scene. Take the scene from Star Wars where Ben Kenobi is teaching Luke how to use the Force while on the Millennium Falcon. Luke starts off disbelieving when he can't see to deflect the training orbs bolts, a - in this case, but when he finally 'sees' the training orb even with his eyes covered this changes to a +.
The other idea he talks about is adding >< at the bottom of the index cards to represent conflict. Drama comes from conflict so if your scene doesn't have any it's going to fall flat. Find the conflict in the scene and write it down here.
The advantage of writing your scenes down on cards is that you can move them around at will. A scene might not work in the place you intended it to go, but it might work elsewhere. It's just a simple matter of moving that card to its new place.
Only when your forty key scenes are completed can you then start to write your script confident you've worked through all of your story's problems. The forty cards will provide you with a blue print for your finished script, and trust me your script will be much better for all that preparation.
Give it a try.