2d & 3d results mismatching

hello sir, i am designing the resonator structure using piezoelectric module. i designed the device and plotted all required results but my problem is that i designed the devise in both 2d and 3d model with same dimensions and boundary counditions but the resultant frequency is shifted much more then it can be???
how to resolve the problem plzzz help.........
is there any one who faces same problem or having the solution???????