I think I'd try digging out a bed where one of them has wee'd, to see if they will use it - if they won't you can always replant with grass seed
mine always use two gaps in my patio, which I keep topped up with John Innes - I find it's quite easy to find the lumps, and the wee disperses with the rain, or I water well in dry spells - they prefer these outdoor arrangements to their litter tray, whatever the weather (and so do I)