Tip: Learn to Feel the Muscle Working

If you don't feel a muscle working during an exercise, you won't stimulate that muscle enough to grow at a maximal rate. Even when training for strength – using heavy weights and low reps – you should feel the proper muscles doing the work. You won't get a big pump in these muscles from strength work, but the target muscles should feel harder after a set.

If you lack the motor skill to optimally activate a specific muscle during a big lift, use isolation work for that muscle to learn to recruit and flex it maximally. When you're good at recruiting that muscle, it'll become more involved in the big lifts. Doing isolation work for a muscle you don't otherwise feel properly is an investment in future gains.

Isolate to Integrate

This is the progression I prescribe for someone who doesn't feel a specific muscle working during a big lift. I call it the "isolate to integrate" approach. Each step should last 2-4 workouts.

Step 1: Learn to isolate the stubborn muscle using isolation work and constant tension, focusing on the quality of contraction. If you can't feel your pecs during the bench press, you can use something like a dumbbell or machine flye for this, or try the D-Roy raise below. Try to fatigue the muscle with the least amount of weight possible on this exercise. That forces you to contract the chest as hard as possible and "find" those pecs.
Step 2: Pre-fatigue the stubborn muscle with an isolation exercise and then do the big lift. The pump created in the muscle will allow you to feel it more during the big lift. This will improve the mind-muscle connection which will help you learn to integrate it during the lift.
Step 3: Do the big lift first by focusing on feeling the once-stubborn muscle. This requires the use of a lighter weight while focusing on proper muscle contraction, not just on moving the weight.
Step 4: Move on to heavier lifting on the basic lift.

Using this approach allowed me to start using my pecs more in the bench press. I used to be all-shoulders in the bench. Despite bench-pressing in the 400-plus range, I had zero pectoral development. The isolate-to-integrate method helped me to bring in the pecs for bigger lifts and better development.