There could be a few reasons why starter is not doubling 

1. Weak or Young Starter

• A new starter (less than 7–10 days old) may not yet have enough active yeast and bacteria to rise predictably.

• Fix: Keep feeding regularly (1:1:1 ratio of starter:flour:water) and wait at least 2 weeks for full strength.

2. Low-Quality or Old Flour

• Bleached, low-protein, or stale flour lacks nutrients for microbes.

• Fix: Use unbleached bread flour or whole wheat/rye (20–30%) for better fermentation.

3. Incorrect Feeding Ratio

• Too much starter left in the jar dilutes fresh food, slowing growth.

• Fix: Try a 1:2:2 ratio (starter:flour:water) for a stronger rise.

4. Cold Environment

• Ideal temp: 75–80°F (24–27°C). Below 70°F (21°C), fermentation slows.

• Fix: Place starter in a warm spot (oven with light on, near a radiator, or use a seedling mat).

5. Underfeeding or Overfeeding

• Underfed: Starves microbes, leading to sluggishness.

• Overfed: Too much fresh flour/water dilutes acidity and microbes.

• Fix: Feed every 12–24 hours (twice daily if warm, once if cool).

6. Chlorinated or Hard Water

• Chlorine kills microbes; minerals in hard water can inhibit growth.

• Fix: Use filtered, bottled, or boiled (then cooled) water.

7. Low Hydration (Too Thick)

• A stiff starter (less water) ferments slower than a 100% hydration one.

• Fix: Maintain equal parts flour and water (e.g., 50g flour + 50g water).

8. Contamination or Mold

• Harmful bacteria or mold can outcompete yeast.

• Fix: If starter smells foul (rotten, acetone) or has mold, discard and restart.

Troubleshooting Test:

• Try feeding with whole rye flour (boosts activity) and keep at 78°F. If it still doesn’t rise, your starter may need more time or a better feeding schedule.