Drop down values do not gets selected.

I have a table

srno (char)

name(char)

last_name(char)

I have created a dropdown list in which these values gets displayed.

The problem started when I entered 10 values, I could access it. when I accessed the 11th value it goes back to the first value. The values that I select has a subroutine assigned to it. By Concatinating the srno_name_lastname.