I was going to say that if they are on backward they will not work. When the injector is working you should feel the suction with your finger.
If is an older one you may have dissolved the spring inside if you used a lot of bleach or acid. The spring will keep the check ball from going all the way up when sucking chem and blocking the inlet port. If this is the case you will need a repair kit to fix it or just replace it for a few dollars more.
Chemical Injectors Design and Function:
Downstream chemical injectors work by means of the venturi principle. This principal states that when water is forced through a restricted area without the volume changing its velocity will increase and it will create a low pressure zone. In a chemical injector a restrictive nozzle is placed just in front of a cavity. When the water flows freely across the cavity without any backpressure it creates an area of low pressure that sucks in the chemical through the inlet barb. This vacuum is strong enough to both draw the chemical and lift the check ball that is in the barb to prevent the backflow of water.
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When the injector is placed in a high pressure mode the back pressure created by the nozzle fills the cavity after the restrictive orifice and eliminates the area of low pressure. With the area of low pressure eliminated the check ball is seated against the o-ring making a water tight seal and eliminating backflow.
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