First thing to try is cleaning the belt itself.
I've done this on nearly every car I've owned - dust and grime on the belt ultimately fouls the mechanism but if you catch it soon enough you can get the retract action working better by cleaning. Just pull the belt out to its full length then using a damp microfibre cloth (only plain water no soap etc) grip the belt on both sides keeping it flat and feed it through the "sandwich" of cloth to remove as much contaminant as you can, 2 or 3 passes should do it. This shifts a little of the dirt from the retract mechanism as well as the belt. And hopefully improves the action,
If it is still flaccid (ooh err) I reckon you're going to need a replacement belt (they're not all that expensive). I definitely wouldn't advise trying any diy repairs on an inertia reel seatbelt.