I know this is an old thread, but I see it coming back to life. I ran into the same problem using Fox Farm Happy Frog, my favorite soil. I posted the exact same question on another forum and one of the experts came in and told me that Spaghum Moss has a tendency to break down, becoming acidic after a few months. I'm not sure if this is true, but my soil PH did drop pretty low during flower, down to around 5.0 which isn't good.
Its also very possible that salts are building up in the soil, especially when aggressive flower feeding is applied.. (a lot of P and K). So, its one of the two things either the Spaghum Moss or too much salts.
I've had good success using this soil, but have found you have to watch PH in the root zone. If you see the salts getting high or PH dropping, a good hard flush* (def 1 below) once or twice during the grow will help a lot.
Definition Flush (there are two used by growers causing confusion).
1. Flush- to run large volumes of water through the soil removing salts. Example, you have 5 gallons of soil, you run 10 gallons of water through the soil (or more) until salts lower. If PH is too low, you can run higher PH water through the soil to help counter the low PH.
2. Flush- To feed plain water at the end of flower, before harvest. This is a completely different thing.
I don't like to flush using either of the definitions, but with Happy Frog I find using definition 1 Flush once or twice during a grow helps a lot.