Although you applied and were granted Part B (outpatient coverage) and pay a premium based on your adjusted gross income on the U.S. tax return, you can now only request Part A during January thru March of any year, and it will become effective no earlier than July 1st of that year. In addition, on top of the monthly premium which can be up to $426/mo., you will be assessed a 10% penalty for each year that you did not sign up for Part A since the year you signed up for Part B. You will carry that penalty in perpetuity.
Please note that when someone does not have Part A, they are solely financially responsible for any hospitalization they incur.
Also, whether you are on Part A and/or Part B, with that eligibility you were expected by Medicare to sign up for a Part D drug plan — which in your case, would have been a stand-alone drug plan. If you did not sign up for a drug plan since obtaining Part B, then during Medicare Annual Enrollment (Oct. 15 – Dec. 7) you can enroll onto a Part D drug plan that would be effective January 1st. You will be assessed a penalty of 1% per month based on the national average for each month since being on Part B that you were not on a Part D drug plan and you will carry that penalty in perpetuity.