Problem with periodic boundary condition

Hi,
i have a problem that i don't really understand.
I made some simulation of a square impedance tube to compute sound absorption coefficient.
The mode works very well.
when i apply a periodic condition for the lateral faces of the tube, the model failed (Failed to find a solution for the initial parameter). The relative error (1e+004) is greater than the relative tolerance).
It can be possible to solve this problem by increasing the mesh resolution bu it work only for some frequencies and the problem get back.
Does someone can explain to me what's wrong?
Thanks a lot.

Jean-Philippe