Lookup or Matching

I don't understand why I am having an issue. I have a column (A) of 4 digit numbers (Employee ID's). Next to it (Column B) I want it to automatically place the persons name there. So in column C I will have the employee ID numbers and in the adjacent cell in column D I will have their names. I tried in column B =lookup(A1,C1:C20,D1:D20). It only worked on two of them. So, A1 ID #'s (Manually entered), B1 formula placing persons name), C1 ID #, D1 Employee name. Columns A & B will continue to grow and reference C & D.


Thanks for any help,
Jay

post a workbook no one wants to build an example to try this out