There's no single answer to the original question. It could have to do with the quality of the cocaine itself, the adulterants used, mood, malnutrition, sleep deprivation, dopaminergic depletion (mentioned above), or something else.
The most likely reason is that you're getting cocaine of varying quality. This is somewhat inevitable because in my experience, the quality of most drugs does fluctuate somewhat. It's not like you're buying it off a store shelf. That being said, the fluctuation shouldn't be tremendous like you're describing. If I were you I'd get a new connect or at least ask for a bump out of every bag before you buy it.