have your tried working in pre exhaustion sets prior to your heavy working sets? this will help you isolate the muscle you plan on working without having to go as heavy as normal, but still allow you to fail in the 4-8 rep range.
example -- you do a heavy bench press, but your shoulders are giving out and failing on you BEFORE your chest is. Yet, you are trying to work your chest and it never gets to a point of failure because of your damn shoulders.
well right before you do your heavy bench press, you pre-exhaust your chest. do 3 sets of high rep flies to failure until your chest is pumped and fatigued, then go right to your heavy bench press (using lighter weight then normal though) . your chest will give out and fail in that 4-8 rep range, and this time it will happen before your shoulders have a chance to fail, because the chest was pre-exhausted.
benefit is your'll get more chest isolation, still be able to go heavy, and take strain off the shoulders.