The most common reason for this is incorrect placement within the existing slots. This is most often seen with either the number of sticks is less than slots or there is a mix of stick densities. Be sure that the memory stick(s) are in the correct slots. As an example, lets assume that there are 4 slots. These are usually bank 0 and bank 1 in the following pattern: 0 1 0 1. If you place 2 sticks in the first 2 slots there will be a stick in bank 0 and another in bank 1. Some motherboards like this, some require the backs to be filled. In other words the two sticks should be in slot 0 and 2 to fill in bank 0.
Check the motherboard to be sure that the sticks are placed correctly.
There are other potential problems, but start as listed above and report back the results.