Adequate coverage on both sides of the leaf surface is crucial for wet sprays to work.
Most insects inhabit the underside of the leaf first since they're more protected there than on the top side of the leaf.
Spider mites are notorious for this, as they feed and breed under the leaf, almost going unseen until their population explodes. The first detection of a spider mite infestation is usually the pin prick-like marks on the top of the leaf, a tell-tale sign that mites have been feeding on your plants.
Ensure to schedule sprays for the early morning or evening hours, so the spray evaporates more slowly and has more time to work.
Also, sprays applied in the middle of the day can burn plant leaves.
And don't forget to spray the main stem and soil surface, too!