This is more complex than just a simple answer, actually. Many factors could cause this, but mine are in full sun, planted at correct depth (varies according to variety), and not being overwatered. On the other hand, quickly warming temperatures can also cause stems and leaves to stretch too swiftly. As well, if the ground or soil dries up, and the daffodil plants start going dormant (returning energy back to the bulb), then, their leaves and stems also start weakening.