Kelp Not Growing after 1.15 update

I updated the Buzzy Bee update on one iPad and then downloaded my world from Realms to play around with new features.  Found that something is wrong with my kelp farm - the kelp is growing erratically - rather, one or two of the plants are growing out of six in a line.  The one plant grows fine and the others does not.  I have replaced the sand, the plants, played with tick speed, checked if day/night makes a difference.