What are some of the reasons why economists have looked beyond the Solow growth model?

Suppose Countries A and B both have 100 workers, and country A has 600 units of capital while country B has 300 units of capital. According to the Solow growth model which country would be growing more quickly (assuming they were not yet at the steady state)? How would your answer change if workers in country A were three times as productive as workers in country B?