why a few number of letters in treeview nodes is invisible?

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C#

Hi I have a treeview that filling with data from database. In the runtime a few letters in the parent nodes are invisible but childs are correct. Example

+ First Par (This is: First Parent)
+ Second Pare (This is: Second Parent)
- Third Par (This is: Third Parent)
Third Parent - First Child
.
.
.

What I do?!!
Thanks

Edit:
my code:

DataTable dt = GetRecords(query);

//parent.Nodes.Clear();

foreach (DataRow dr in dt.Rows)
{
TreeNode tn = new TreeNode();

tn.Name = dr["groups"].ToString();

tn.Text = dr["text"].ToString();

tn.ForeColor = Color.Green;
tn.NodeFont = new Font("Times New Roman", 11, FontStyle.Regular);

treeview1.Nodes.Add(tn);

}

i found that problem is in the font changing, when this line is commented displaying is ok.
this problem only in parents was detected and if i using this code for fetching childs nodes, no problem detected (with font changing).

in the disign i set the treeview1 font on "Times New Roman" size 10, if help!