So its the location of the tray not the litter
Definitely, location, location, location - everytime LOL!!
Mine have got 4 trays in the walk in cupboard under the stairs in the kitchen (other trays are elesewhere around the house) and the 2 trays at the far end away from the door are the most used trays, (which of course is the most difficult place to get at coz of the sloping ceiling!), the 2 nearest the door are hardly ever used.