I've had dahlias in the garden for two years now, and this summer they barely produced any flowers. I water them every two days because it gets brutally hot here in August, they get direct sun from 7 in the morning until 5 in the afternoon, and I use fertilizer every 15 days. I don't understand what's going on - in previous years they'd be covered in flowers.
I had something similar happen with mine, and it turned out I was watering too much, which rotted the roots without me realizing it. Every two days is pretty frequent, especially if the soil drains well or it's been raining; dahlias prefer to dry out a bit between waterings. Another common factor is soil depletion after two years without changing the soil or adding stronger nutrient inputs, because fertilizing every 15 days maintains it but doesn't replenish it. I'd recommend reducing watering to every 3 or 4 days, checking that the soil isn't compacted or waterlogged, and if you haven't changed the substrate in two years, maybe it's time to do it or at least add compost or a more phosphorus-concentrated fertilizer to encourage blooming.
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