All the tadpoles died after a few days for no obvious reason.
There is most likely to be a problem with the feeding of the tadpoles. You can find information about breeding at ‘mating and breeding‘ on this website.
A lot of the tadpoles died after a few days but the others are growing normally.
This is a normal process. Nature ensures that the tank is not too densely populated. Daily water changes of 30 percent can help. If there is no filter in the tank you have to change the water more frequent.
Some tadpoles are growing faster than others.
This is a normal process, too. Nature regulates the availability of food for the tadpoles. In most cases you will have what appear to be two generations of tadpoles from one batch of eggs. The smallest will take double the time or more to develop. When the older tadpoles have finished metamorphosis, they will continue growing fast as before.
African Dwarf Clawed Frog tadpoles won’t eat the frozen food.
Firstly, young ADCF tadpoles don’t eat food from the bottom of the tank. Secondly, the food must not be too large for the tadpoles’ mouths. You can drop frozen Cyclops or Bosmids directly onto water plant leaves or sticks so that the food stays higher in the tank and doesn’t drop immediately to the bottom. To clean the water you can put some dwarf corys (Corydoras pygmeaus or Corydoras habrosus) or some small snails into the tank. They will eat food left by the tadpoles.