Apply antiperspirant at night, as that is when they work best. The active ingredient has to go down into the sweat duct and clog it. If you apply it in the morning, when sweat volume is typically higher, it often washes off.
Applying antiperspirant at night to dry underarms also reduces the chance of skin irritations and will help keep you using it more faithfully. Consider drying underarms with a blow dryer on cool setting before applying, then lightly drying after applying.