You would have used wrong type of yeast which means the quantities would change causing the use of too much yeast giving out yeast smell. 2. Your oven was not pre-heated, so the yeast was not activated and the dough remains undercooked and raw in the centre. 3. Oven was too hot, killing the yeast so the challah remains dense. 4. Watch our bread baking queries video to understand more in depth about various bread related problems.