EMB-170 Captain reported that the Flight Attendants had difficulty getting the door arming lever to go past the half way point. Despite the door being disarmed; they noticed the escape slide was partially deployed. Maintenance was called.

Narrative

We were waiting for the doors to close when the FA A said FA B could not get her arming lever to go past the half way point on the R2 door. FA A then went back to the aft galley to try and help. She called up to the flight deck and said they were unable to get the arming lever to go down. I said it could be an alignment issue with the girt bar. I told them to both verify both the arming lever and vent flap were up as well as verifying the door was disarmed and try to realign the door. They both made sure the handles were up and the slide was disarmed before moving the main door handle. They then noticed the slide was partially deployed after moving the main handle. They told me it was a live door; so I told them to get away. I called operations to bring the jet bridge back and maintenance. One of the maintenance personnel said to FA B he believed the slide was already blown before moving the main door handle. I also spoke with FA B and she said her preflight checks were normal and she never touched the main handle till that moment. The FO also noticed no irregularities on the walk around. Cause: Unknown. Suggestions: Should have had maintenance look at the door when they could not get the handle to come down.