SQL LAG() function returning 0 for every row despite available previous rows

Question

I'm using the LAG() function in SQL to calculate the difference between the current month's tips sum and the previous month's tips sum for each taxi driver. However, the LAG() function is returning 0 for every row, even though there should be previous rows to reference.

This is the example of the query I am using now:

    SELECT 
    td.taxi_id,
    td.year,
    td.month,
    td.tips_sum,
    (td.tips_sum - 
    LAG(td.tips_sum, 1, 0) OVER (PARTITION BY td.taxi_id ORDER BY td.year, td.month) /
    NULLIF(LAG(td.tips_sum, 1, 0) OVER (PARTITION BY td.taxi_id ORDER BY td.year, td.month), 0) AS tips_change
FROM 
    tips_data td
ORDER BY 
    td.tips_sum DESC
LIMIT 3

Example of the result I get right now