Not letting your body acclimate to the heat. Summer workouts effect your body differently. In warmer conditions, your body temperature increases faster, making you weary faster.
Your water intake is low. When you sweat, salt is released. You must replenish sweated fluids to avoid dehydration.
Your fuelling is off. Food provides energy for a good workout. "Eat a full, nutritious meal several hours before working out.
You're dressed incorrectly. Many assume wearing layers or heavier garments that make you sweat would give them a better exercise.
Not safeguarding your skin. No matter how long you workout, bring sunblock. Sunburns can occur in 15 minutes.
Pushing oneself too hard. Don't give up if the heat makes your training difficult. If you can't keep up, adapt your workout until your body adjusts to the conditions.
Your workouts are mistimed. Working out in the sun noon is unwise in hot, humid weather. Try early morning or late evening workouts.