DataTable value not reflecting after some loop in C#

Question

I am using for loop and it's looping number of ".mdb".

certain period of loop, i am getting null values in Data Table.

If we say exactly, after 19th count (0-19), 20 loop values will becoming in null values in Data Table.

How it does come and what will be the reason.

below is my code.

String EmpNo = String.Empty;
for (int i = 0; i < LISTmdb.Items.Count; i++) // More than 100 mdb
{
    EmpNo = GetEmpDetail(ds, mdbPath);
}

public static int GetEmpDetail(DataSet ds, string mdbName)
{
    string tblename = "EMP_SUMMARY";
    DataTable tblRes = ds.Tables[tblename]; // After 20th count, "tblRes" will be showing as null...?
}

but, In our MDB data's will be there always and this has been checked.

why it's getting null value in DataTable "tblRes" after exactly 20th count.....