I had this problem; however, found that it was because i was taking short ri’s (ie: 1 minute) between each set of benches (when i started lifting, i was under the impression that shorter ri’s were better; which they are in some cases; but not always).
As soon as i increased the ri’s to 3 minutes;my bench shot up (using heavy wieght 6 reps or less). Not sure what your ri’s between benches are; but if they are not long enough; it could be that your chest does not get enough recovery time; and, therefore, other muscle groups come into play to compensate. If your ri’s are already 3 minutes or more, then the problem is something else.