If I used my vehicle for work more this year (2021) than last (2020), why am I not getting the deduction that I got last year?

Mileage deduction is 1.5 cents less per mile in 2021 compared with 2020. 

Are you taking standard miles or actual expenses? If you are using actual expenses, then there is a proration between your total miles and actual miles that determine the amount of actual expenses you can take. While you may have driven more business miles, you may have also driven more personal miles.

Thanks for your response. That's what I thought, but it turns out I drove fewer miles overall in 2021 than in 2020, but more of those miles for work. So, a larger proportion of the total miles was for work in 2021. I wonder if there are now a minimum miles that must be reached before the deduction kicks in? 

BTW, I use the odometer reading for total miles, and I track work miles with Quickbook.