Squeeze the bellows/gaiters at either end of the rack.
If they soumd squishy or have evidence of oil at the ends, that's where your oil is going. The bellows shound be dry inside.
If they're full of oil the rack needs a set of replacement O-rings. Now, don't ask me if they are available as I suspect manufacturers would rather sell you a complete new rack these days.
Worth a bit of Googling though, the difference in price between a couple of O-rings and a new rack is likely to be a lot!