For the absence of birds in such a small area as a garden there are many reasons. The first is that individuals we see around the house in summer mostly don't stay with us in winter either, but wander a few km or tens of km around. To our feeders come birds from elsewhere.
The second reason is related to the first - as said, birds wander around in winter, so it's possible that there are fewer one year, more another. For example, there are really a lot at ours this year.
The third is related to the warm weather in recent days. Birds always prefer food in nature (feeders usually represent at most 10% of all their diet) and since in these warm days they find enough, they don't visit feeders so often.
If there is any more question, I'll gladly answer
