Could be the problem is due to the two seat bases being not exactly parallel - usually wider at the end away from the chest of draws. measure carefully, and if this is so, get some wooden moulding in the form of the segment of a circle from DIY and stick along the tracks, thus taking up the gap. may not be necessary all the way along. stain these down with some wood dye and you wond notice them. This will prevent the slats moving sideways enough to fall off the track, which may be the cause of the problem.
If we could include pictures I could show you much more easily than trying to describe it, but I hope you will get the idea.
Then, as others have said, you need to hold the base uder some tension lengthways, so fit a small block at each side, such that the solid part of the slat arrangement fits over them and is prevented from retracting.