why is it that a genotype of an organism can always tell you the phenotype but a phenotype can only sometimes tell you its genotype?

Sorry for the long answer ... Suppose that T is a gene that produces tall tomato plants and t is a gene that produces short tomato plants. So if we know the genotype (TT, Tt, or tt), we can determine the phenotype

Explanation:

TT and Tt are both tall tomato plants, but tt is a short tomato plant. Jus because it's tall, we don't know if it's TT or Tt.