phononic crystal -- Acoustic Structure Interaction

hello,

i'm computing the phononic crystal band structure using the Acoustic Structure Interaction modal, and the PC is 2D
square lattice and the unit cell is a circle in a square area. the problem is that when the square is liquid and circle area is solid, the band structure is good. but when the square is solid and circle area is liquid , that's to say, the liquid cylinder in the solid base, the computing result is worse, besides the wanted band , there are many flat bands , which are like the locally resonant mode ,
i'm troubled with it.
can anyone help me solve this problem, and give me a way to remove these flat bands

thanks!!!!