The cardinal rule of A/C service is "CLEAN first, GAS last". If your condenser coil is dirty or full of grass clippings the system can't preform to maximum efficiency. Adding refrigerant to a dirty system is self-defeating and the worst thing a service man could tell me is, "It just needed a little gas".
The refrigerant runs in an enclosed system, changing from liquid to gas and back to liquid again. It doesn't get used up, loose its efficiency (wear out), or evaporate away unless there is a leak. Therefore, if you tell me "it needed a little gas", I want to know where the leak is and what he did to repair it.
Another problem we sometimes see is too much shrubbery too closee to the condensing unit. You may want to check that out too.