Each gun/choke combination may have a different "best trap load". If you have not done so, pattern the gun with different loads (brands, shot size, velocity) and chokes until you find one you are happy with. Also, before shooting patterns make sure the point of impact is where you think it is by shooting a few shots at 15 yds or so. Your gun might not fit or it might have some other problem that is causing it to not shoot where you are looking/pointing.
If you are shooting a trap gun, find out how high it shoots (ex. 70% above where you point, 30% below, etc). If you are shooting a field gun, it can be tough. Field guns generally shoot "flat" and the target is rising for the first 20 yds or so in trap. So, if you want the target centered in your pattern, your choices are 1.cover the target with your muzzle (essentially you can't see the target when you shoot because you are pointing above it) or 2. wait until the target peaks or starts to fall. Either way is a real challenge to most people.
Once you have established that the point of impact and patterns are sound, you only have a couple other hundred things to blame poor scores on
The saying goes that trap shooting is 90% mental and 10% mental. Best advice I ever got was to 1. develop a routine and stick with it for each and every shot, and 2. only think about breaking the next target. If you start thinking about the one you missed or how many straight you have hit, you are setting yourself up for a miss.