Make sure the nut(Slide Arm Cap) that holds the forearm on is not loose.
The disconnector may have some debris that causes binding, access to it is made by removing the 'Slide Support, Left'. Tread carefully in this area, lots of parts can come loose once that Slide Support is removed.
To get to the Slide Support the whole trigger guard assembly must be removed. At this point you have started field stripping and other major components may shift out of position. Consider learning the field stripping for cleaning procedure prior to attempting internal repairs.