Injectors very rarely go bad, so the usual cause is an incorrect pressure differential. Injectors create a vacuum, known as the Venturi process, that allows chemicals to be drawn from the container, mix with water, and delivered to the applicator. There needs to be 200 PSI into the injector and 60-80 PSI on the backside.
Tubing size and injector size can cause incorrect pressure differential. If the poly-flow chemical tubing is too small a "HIGH back pressure" situation will occur -- most low to medium flow applications require 1/2" OD poly-flow chemical tubing, but larger high-flow applications will need a larger BAM port and 1" ID hose. To determine the correct injector size needed, count the number of nozzles and GPM each delivers to arrive at total GPM's for the application. If the application contains air, the injector needs to deliver 1/3 of the total GPM's. If the application does not include air, the injector needs to deliver 1/2 of the total GPM's.