I've been reading this, and trying to figure out exactly what you mean. I interpret you quest in 2 ways: A) how to say that something was within the threshold, but then dropped below it, and B) how to say that something never reached the lower limit of the threshold at all. The first has me more stumped. The second could simply be expressed "The threshold was unattained/unreached/uncrossed". I like "unattained". This does fit your original example, but I still can't tell if it's what you mean to say, or if you mean choice A...