They should have 2 gas struts. These do often leak and fail and sometimes the brackets where one end mounts under the side bolsters on the rear seat break off. About $20 for replacement gas struts on ebay e.g.
http://www.ebay.com.au/itm/2-Gas-Sta...item5d59441d00 but 1st check you still have the brackets under the rear seat bolsters.