seed popping out while planting

while playing on a new world on 1.9 that got released today 2-29-16
im in a mooshroom biom underground i am planting wheat seeds on a 9x9 farming plot and in certain areas of the plot seeds will pop out after planting them i have tried crouching, standing on outside blocks and they still pop out. when im planting wheat seeds either one will pop out or 2 side by side will pop out