The old ones tend to fall out of place quite easily.
There is a little retention clip in the middle - which is essentially just a pin that holds in a slot on the runners, but this gets worn and stops it holding in place.
New ones are quick and easy to fit, its always tempting to grease the guides, but there's a big risk it will dirty the carpet - so use it sparingly, thin lubricants like WD40 tend to be a big naff for stuff like that.