With USB charging it gets complicated as there's circuitry controlling what the charger outputs (which can depend on what it sees as a load), and circuitry in the device being charged that decides what's acceptable and what it will accommodate. There's likely a threshold that's needed before the device can effectively start charging something, and that threshold will depend in detail on the characteristics of the charger and the device it's trying to charge. Perhaps you weren't hitting that threshold for these devices.

Some solar chargers get around this by having their own battery. They design the charge circuitry to efficiently charge that battery even at low light levels and when that battery has sufficient charge, it is then used to output a useful voltage and current to charge a connected device.

Note that LightCharge offers an aux battery pack that they recommend to use between the dynamo charger and an iPhone. It's likely this pack functions in a similar way to the solar charger battery I described.