There are many reasons for a 'short fail', too many reasons to name. For example, here are a few scenarios for the short relief in my clinical experience:
1 - The patient's diet and lifestyle are not maintained in a strict enough concordance for proper healing and restoration of the gut to proceed (and to finally overcome candida/parasites, etc). What I mean is, people are people and will be "good" for certain periods of time, but then succumb to being "normal", allowing the pathogens to get the upper hand once again.
2 - Pathogens live in cycles, the breed/multiply/live/die, etc., just like we do as humans - but in much shorter cycles Sometimes there will be more, at other times less. They also live in highly competitive relationships with other pathogens/commensals and beneficials, and all this is also determined to a large degree point # 1 I just made.
3 - A person's immune system activity is most variable indeed, and is largely dependent on exactly what is going on is his/her life at any given time. Cortisol (made by the adrenal gland) is one of the most important of the core hormones to influence both the cell mediated and humoral immune response. And guess what? As soon as any stress occurs, cortisol production will wax and wane like the moon, i.e, it will go up and come down. And so will your immune system's activity. So, what I'm trying to tell you is that your immune system's activity is NEVER the same when it comes to fighting any internal bugs.
4 - People react to herbals and nutritionals differently, some need more (in fact, a LOT more than they take) and others need less, a LOT less. I've discovered from experience that NO two patients respond the same to an antifungal. This is like saying that ALL men go to Walmart and are satisfied with buying a pair of size 16 boxer shorts that are white. LOL, some guys like yellow size 8, whereas others hate boxers and only buy briefs, etc. It's all about individuality, and each and every patient needs to find the right key (dosage, herbal, etc) to unlock their antifungal door.
These are just four of the one hundred variables I could write, you are probably starting to get my point...