Usually this is due to one of two things. (Assuming you train correctly and efficiently with a great eating plan, and allow enough time between workouts for full recovery)
1-You triceps are overly developed causing them to become the dominant mover during your chest workout. This results in an ineffective stimulation of the pectoral muscles.
A sometimes effective solution for this scenario is to pre-exhaust your Tris with an isolation movement before your pressing movements.
2-Your triceps are underdeveloped so they fail out early, causing a cessation of the set prior to your chest receiving efficient stimulation. This again results in an inadequate stimulation of the pectorals.
A sometimes effective solution for this scenario is to pre-exhaust your Pecs with an isolation movement before your pressing movements.