I let them callous over for a few days beforehand.
They take a lot longer than 2 weeks to begin rooting, in my experience. But if you leave them in the water they’ll root eventually.
As a side note, the babies will not have yellow leaf edges. This type of sansevieria reverts to an all-green variety when propped this way. If you want to keep the yellow edges, you have to prop them by letting them bud off the main root.