The first two times I tried this recipe I had the same experience you did – no rising, but a fantastic tasting bread. This week I read that 50% hydration starters take much longer to rise, so I let the bulk fermentation extend to 5 hours and the shaped loaf proofed for nearly 4. During both cycles, to my surprise, it would up doubling! And, the best part was that it had fantastic oven spring once baked.