There are three types of recliner backs, locking levers, locking tabs, or catches and bolted. When fixing locking levers, check the brackets on the sides and make sure they're fixed. They need to be flipped down and locked onto the seat's metal posts.
The locking tabs snap into place when you slip the back on. Remove the bracket fasteners and tighten them to secure the back. Use a flat screwdriver to ensure the tab fits in the slot after the back is fitted on.
With bolted on backs, loosen the lower side of the back panel. Remove the bolts underneath and refasten them by ensuring they're tight and hold the back to the seat posts.