I have had this issue for a while now, and it baffles me. I've had the Stenner 100 psi 3 gpd pump for a little more than a year. Installed in May 2015. Worked well initially, but now I am convinced it's not injecting into my main line downstream of my filter. Here are some specifics:
1) Pool filter operating pressure is about 20 - 23 psi. Stenner is a 100 psi pump.
2) I have a new check valve in the injection head.
3) When I simply run the discharge tube from the Stenner into a measuring cup, I get 8 oz of bleach in the cup in about 30 mins...the equivalent of 3 gpd.
4) When I hook up the injection head and repeat step 3, i get a little less than 8 oz in the cup, but still, it's pumping fluid.
5) My timer runs the Stenner pump about 3.5 hours per day, so about 0.5 gals of bleach per day.
6) I made a DIY measuring stick for my 15 gal chemical tank. I can check it over several days and the fluid level does not go down, so when the injection head is connected to the pool, its not moving any fluid.
So, the issue appears to be that the Stenner pump cannot buck the 20 - 23 psi pressure in the pool system. My injection head is installed using a T connection, and I did not trim anything off the injection head. So that little extension downstream of the main injection head body is intact and probably ends right in the middle of the flow stream from the pool system.
Needless to say, I've got all kinds of issues with managing my FC level. Being a new Stenner pump owner (relatively), I'm not sure if this is an issue with the #1 pump tube inside the Stenner pump housing. I have never replaced it yet (but i do have spares), and I don't know if there is a connection between the lack of injection against 20 - 23 psi, and the mechanics of how the Stenner moves fluid through the pump tube.
I found a thread that described a similar issue, but I think I have evidence that the Stenner is not moving any fluid, since my dip stick says that the fluid level stays the same in spite of the pump running and presumably injecting 0.5+ gals per day.