Pine Siskin is known for being irregular in its winter habits. Reduced numbers in the short term do not necessarily mean the species is declining overall. Wikipedia:
Migration by this bird is highly variable, probably related to food supply. Large numbers may move south in some years; hardly any in others. This species is one of a few species that are considered "irruptive winter finches" because of the high variability of their movements based on the success of crops from year to year.