Check/Uncheck All, write conflict

Hello,

I have a form +subform, on the subform (countinuous form) I have a check
box. on the main form I have an unbound check box that I want to
check/uncheck all the check boxes of my records on my subform. I have an
update query to perform that task.
-Problem 1: after ticking/unticking the check box on the main form, I get
all the check boxes on my subform ticked except the first record where the
focus is.
-Problem 2: I get a write conflict message " Save records or Copy to
clipboard or Drop change "

Can someone assist ?

Many thanks.