Sounds as though there may be something eating the roots. If it's an indoor yucca empty it out of the pot onto a large piece of paper and carefully search for a white grub. Vine weevils eat roots. You will need to squash them all and then plant into new compost. All pots will need to be treated. If they are outside yuccas you have a bigger problem. I would dig up the remaining yuccas and search the roots for weevils then replant elsewhere. Of course it may not be vine weevils. Good luck!