Where is your barrel when you call for a target? Is the bird coming out under your barrel? Are you a one or two eyed shooter? These questions all have a bearing on what your problem may be. If you hold your gun over the top of the trap house, is it in the center, or is it off to the side a bit? If it is above the front edge of the house, and centered on the house, try holding a little to the right, presuming you are a right handed shooter. This is important if you are a one eyed shooter,as a straightaway coming out under your barrel will surprise you when it pops into view, and if you get a glimpse of it before it is truly clear of your barrel, you will usually lift your head to see it more clearly. Lifting may not be a gross movement, but may just be a subtle change of head position on the stock. In the same scenario, if you are a two eyed shooter and it comes out under your barrel, your left eye may see it first, and try to pick it up by becoming the dominant focusing eye, which throws off your sight picture relative to the back sight (your right eye) and the front of the barrel, guaranteeing a miss.
To begin to find a cure for your problem, first pattern your gun to see exactly where it shoots. Next, practice on a machine set on straightaway, and try holding just below the lip of the house. Keeping in mind how your gun patterned, track through the bird as you shoot. If you keep your head welded to the stock, your straightaway scores should improve. If your gun shoots relatively flat, remember that it is a rising target, and you will have to get close under, or maybe even cover the target, depending on how quickly you shoot. If it shoots high, you can float the target a little. Take the extra second each bird to review the basic points you are trying to instill in your shooting, call and shoot the bird, then review your shot to see how you did. Did you hit it squarely and smoke it? On top and drive the pieces down? Underneath and drive the pieces up? Remember the sight picture on the dustballs, and try to copy that picture every straightaway bird. Good luck.