If you lost your subsidy eligibility it may be because of changes to your annual income, your zip code, or your taxable household size.
Since subsides are determined specifically by the above factors, any change, big or small, can have an impact on the subsidy amount and whether or not you will get a subsidy at all.
In certain circumstances if your income was higher than in previous years, you may have passed the threshold that would allow you to receive subsidies. In this situation the government has determined that you are earning too much to receive assistance and will offer you full priced plans.
On the other end of the spectrum, if your income went down to the point where you qualify for medicaid, the government in this situation has determined that at your income level your needs would be best served by receiving medicaid coverage. In this situation you become simultaneously eligible for medicaid but ineligible for premium tax credit subsidies!
You can find more information on how subsidies are calculated —> here