trouble: checkbox in matrix

Hi, friends!

I can't solve the problem, please, help!

I've designed a form in ScreenPainter, which include

only matrix with 2 colums: checkbox-column and OITM.ItemCode-column.

All I want is to select ItemCode throught checkbox-cell.

I click over the checkbox, its become checked,

<b>but</b> it become unchecked when i scroll down

the matrix and then scroll up to my checked ItemCode!!

What does it meen?

Thanx, Alex!

(Excuse for my EN.)