My custom serial number reach number 10 then stop

[Bright1Light]

Hello all

I have create custom serial number but it stop when it reach 10 I don't know why

I have button generate this expression

Me.txtSerial*= Nz(DMax(“[Serial]”, ”tblOrders”, ” [City] = ‘“ & Me.[txtCity] & “’”),0)+1
Me.Dirty*= False

Could you please help to fix this expression..