Slow-fermented bread doesn't rise

Question

I've tried the slow (or cold) fermentation method of baking bread at home and for the first time it turned out to be very nice. But ever since I use this method the dough just doesn't rise that much and each loaf ends up flat and not very fluffy. The method requires the dough to be leavened in the fridge for 12-24 hours. I've used hand-bake yeast that I activate before making the dough. What I have done so far:

• add more sugar
• make a small batch of dough with half the flour and no salt, then the next day mix it with more flour and salt
• add less salt
• leaven the dough for half an hour BEFORE it goes to the fridge
• put the dough in the fridge straight away
• leaven the dough for a few hours after it was taken out form the fridge
• put the dough in the oven for half an hour on the lowest temp setting

None of these helped. Any ideas what have I done wrong?

Answer 1

UPDATE: recently I started using drier starter with bread-dough like consistency and add it when it's about room temperature. Measuring the ingredients precisely also helps, provided you figured out the proper ratio of flour and water.

My solution: simply use more starter. Generally the starter is kept in the fridge in a jar. I take out the jar from the fridge, add around 150g of flour and 150ml water. After an hour or two, once there are bubbles on the surface, I just add around 300g of this mixture to the bread. The jar (with little starter left) goes back to the fridge. The