Is your Meyers lemon tree in a pot or the ground ? if it is in a pot you probably have fungus gnats. part of their life cycle is in the soil of the pot. so when you spray you only kill the ones you spray. Insecticide soap is a contact spray so you will kill only the ones you spray. The soap on the leaf will not kill the insect. To control the larva in the potting soil letting your pot dry more before you water each time. Let the lemon tree just start to wilt. keeping the soil drier will reduce the gnats.
Look in your local stores for products that control Fungus Gnats.
Contact your local Extension service office for more possible solutions.