Grid convergence 2D Wave tank
 

I am running a case for a 2D wave tank with the following properties:
Length of domain =21m; height of domain = 4.65m. The wave characteristics were also revised as follows:
Wave height = 0.03m
Wave period = 2.5s
Wave length = 9.44m
wave model -Stokesfirst
Size of cell used is 750 x150
Aspect ratio = 1

The free surface elevation for the numerical model from OpenFOAM does not correlate well with that of the theoretical model. Can anyone with experience on this say why there is no convergence? Is something wrong with my AR or cell sizes?