Why did i get a tax credit last year from Medicare part B and not this year?

Medicare Part B is a health insurance premium that is a deductible medical expense.  It is a itemized deduction not a credit. How it works is that that if you are able to itemize your deductions that exceed your standard deduction amount for the year, then you can claim medical expenses that exceed 7.5% of your Adjusted Gross Income. Medicare premiums are deductible medical expenses if you are able to take advantage of this deduction.

Based on this fact, you may have been able to itemize last year while this year you were unable to.  This is the only reason I can think of. The only other thing that comes to mind is that if you were enrolled in the Affordable Care Act last year, you may have received a Premium Tax Credit if your estimated income was less than you projected for the year. You may have mistaken this for Medicare Part B.