Could you have overwatered it? You can check by tipping it out of the pot and seeing if the roots have turned brown and limp or maybe even rotted away.

During the winter at comfortable room temperature they need to be kept barely moist, even dryer than a Clivia, if you have grown those.

Keiko

I agree with Keiko, but I deal with the same thing by making sure they have a super drainage soil. Something with enough drainage that the water will just pour straight through & not pool on top. Mine is just about fully dry after a week. I received 3 more of them recently from someone that was having trouble with them I immediately repotted them into a much better draining soil, but they yellowed & lost their last couple leaves like yours. I'm guessing it was becuase their roots were rotten from overwatering so they didn't deal with the transplant so well. The bulbs stayed firm though, & a month or so later I've got nice pink swelling buds on the top of all 3. :)