A starter that won't rise is almost always fixable — there's exactly one situation (mould) where you start over. The rest come down to five causes, and the most common one by far is simply that your kitchen is too cold. Here's how to diagnose and fix each, roughly in order of likelihood.
First, know what you're aiming for
A healthy starter doubles (or more) within about 4–8 hours of feeding at room temperature, is bubbly throughout, smells pleasantly sour, and domes before it falls. If yours isn't doing that, work through these five causes.
1. Your kitchen is too cold (the #1 cause)
Wild yeast is highly temperature-sensitive. Below about 65°F (18°C) fermentation slows to a crawl; below 50°F it nearly stops. This is the single most common reason, especially in winter or air-conditioned kitchens.
2. Your feeding ratio is off
If you keep a small amount of starter and feed it a lot (like 1:5:5), a weak culture gets diluted so much it can't work through the food and never peaks. For a struggling starter, switch to 1:1:1 — equal parts — so the existing culture has the most strength relative to the new food.
3. Your water is working against you
Water above about 110°F kills the yeast outright — never feed with hot water. In a cold kitchen, slightly warm water (78–85°F) gives the yeast a helpful boost. Heavily chlorinated tap water can inhibit a young or weak starter; if you suspect it, use filtered water or water left out overnight.
4. Your flour is holding it back
Bleached flour can slow fermentation. Switch to unbleached, or add about 10% whole wheat or rye — whole grains carry more wild yeast and minerals and often jump-start a sluggish starter. Switching flours constantly also makes timing unpredictable; pick one and stick with it.
5. It's rising — you're just missing it
This is the sneaky one. Many bakers think their starter isn't rising when really it peaked and fell before they checked. If it peaks in 4 hours and you look at 8, you'll see a flat, deflated surface and assume nothing happened. Mark the jar and check more often, or feed at a higher ratio (1:3:3 or 1:5:5) to stretch the peak window so you don't miss it.
A quick diagnostic
Feed at 1:1:1 with slightly warm water, keep it around 75°F, mark the jar, and watch for 4–8 hours. If it doubles, it was a timing or temperature problem and you're fine. If it doesn't double after a few days of consistent warm 1:1:1 feeds, it needs more time to establish — a new starter can take 10–14 days to get strong.
Sources and method: see our methodology and references.