would need to see the log to know for sure. Can you paste that or take a picture and post it? One issue you might be having is the name of the file you are posting. If it is not the same as what the action is tied to, then the "original" file will not get updated and thus the import of the new data will not work.
For example, when the model builder originally defined the action, it was "tied" to "Actuals_Jan.csv". Well, if you are now loading "Actuals_Feb.csv", the action will run, but it will run the file from Jan which is why we do not advocate have file names that could change. In order to see which actions are tied to which source, in Anaplan, go to Actions and choose the Import Sources tab