During a dynamic calculation the user should set the amount of additional steps and the amount of dynamic substeps such that he/she obtains the desired time step. Also see this related article for more information on this subject: Time step used in dynamic calculation.
For calculations with a dynamic (point or surface) load and more than 1 dynamic substep the results may be incorrect. This is caused by the fact that the dynamic load vector is incorrectly only updated at the start of the first dynamic sub step and not throughout the other dynamic sub steps. To what extend the results are incorrect is dependent on the situation.
Please note that when working with dynamic prescribed displacements (such as for earthquake) simulations the aforementioned issue does not occur.
When using the PLAXIS 2D AE version it is recommended to explicitly set the dynamic substeps to 1 for the situations when working with dynamic loads.
This issue is solved in the latest 2D2015.00 release.