Answer: There are really only three things that will prevent daffodils from blooming and you've covered the most common one. The other to consider is whether or not the bulbs are overcrowded (doesn't sound like it, but it's worth some thought). Overcrowded bulbs compete with one another for adequate moisture and nutrients. The final cause is if the bulbs are planted too deeply they'll run out of energy just getting to the soil surface. I'd dig the bulbs this fall when they are dormant, work some bone meal into each planting hole and reset the bulbs only 2" below the soil surface. This should do the trick.
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