I've always had the opposite problem too, I can do 8 reps with 275 any day, but I can only get 315 up on a really good day! What fails your triceps of pecs? My triceps always seem to be my weak point, so I always do a triceps isolation exercise after I finish benching and that seems to help the next time I go to bench.
If upping your reps on lower weights is a goal, try one of these:
1. pick a wieght you can just do around 15 reps with. Once a week instead of your normal chest routine, just do 2 sets to complete failure. Add 5 pounds every week until you can't get at least 10 reps any more. Then start the cycle over again with original weight plus 5 pounds.
2. Pick a weight you can just do 12 reps. Do 1 set to total failure, then reduce the weight around 25% and rest 30 seconds. Do a second set to total failure (you should be able to do 15+ reps with this weight and your pecs, shoulders and arms should be burning by the last few reps). Then go back to the original weight and rest 30 seconds and do a final set to total failure (you should be able to do 6-8 reps on this set). Next session try to add 5 pounds to the weights. If you can still get at least 12 reps on the first set then add more next workout, if not stick with the same weight. This is also a great way to get a very intense chest workout in <5 minutes if you are rushed for time.