There is no reason it should not allow it. Check a few things.
Be sure the check box is Enabled
Be sure the check box is not Locked
Also, If you are trying to tab to it, be aware you cannot tab from section
to section. for example, if you have 4 controls in the detail section of the
form, it will cycle through all the controls in the detail section 1, 2, 3,
4, 1, 2, 3, 4, 1, etc
You will need to use the mouse to click on it directly.