Amazon Fulfilled prime product just lost next day delivery offer

Hi,

As title stated that couple of days back i noticed i have 9 units left for buyer, i was shocked because upon checking inventory management section i had 900+ units available in FBA and 40 in reserve.

Yesterday those 9 units sold now next day delivery option is removed from my listing. never happened this before

The last shipment i received in full and closed in June. so there should be no issue. my other products have next day delivery option. i am losing my sales and seller support is not helping. is it because of some glitch or error?

what could be the reason if anyone can help i would be grateful.

Thanks