You are not explaining how you mix. Are you using a powerhead allowing for at least 24 circulation? This is essential for healthy salt water mix as well as adding buffer.
I suggest you set up a dedicated Rubbermaid type container (I use a tall kitchen garbage can), place your fresh water in there with a powerhead that can provide good circulation throughout the container. (I like to place the powerhead on the bottom of the can with the output facing upward. You should be able to see a gentle circulation at surface). My preference is to allow for a salt mix prep of 48 hrs, but it can be done in 24. At the end of the 24-48 test for pH, adjust accordingly, then allow the buffer to circulate at least for another several hrs. Test pH. You should never just mix up salt, buffers, any granular additives really, on the quick. They must be thoroughly mixed and aged to get accurate readings (especially so when adjusting pH). Salt already has buffers so you may not even need it.
Do you test your specific gravity in the QT tank itself? What about testing pH? What are the reading from the QT? Your refract should also have ppt readings....please post that measure as well.