Hi there
It could be a number of things cause I don't know what you've already tried his a few things for instance you could try,feeding it more it to more flow or less flow and also They recommend it should be placed in the sand also they don't always open fully every day Some times only half the size and one more thing have you noticed anything picking at it night all day,this would also affect its size this is just a few possibilities no doubt there are many more good luck