There are multiple possible explanations.

The withholding tables are based on the income from your current job (the one W-2). Thus if you have one job, the withholding will be appropriate. But if you have two jobs such that the income from the second job pushes you into another tax bracket, then neither withholding will have been sufficient.

Furthermore, a number of items are depending on your Adjusted Gross Income - the more W-2s, the higher the AGI. But credits like the Earned Income Credit start getting smaller (until it disappears) as your income increases.

We can't see the rest of your question, but these are likely reasons why your refund dropped.