Add grid_columns and grid_per_click parameters. Example:

[stachethemes_ec grid_columns=4 grid_per_click=4]

grid_per_click parameter defines how many events to load per "Look for more" button click.

That's because there's not enough space to contain the boxes.

You can stop the code re-calculating the columns by opening file stachethemes_event_calendar\assets\js\front\stec.js and comment lines 4429 to 4435:

/*
                    while (($instance.$boxgrid.find('.stec-layout-boxgrid-events').width() - ((parent.columns - 1) * parent.options.gutter)) / parent.columns < parent.options.blockMinWidth) {
                        parent.columns--;
                        if (parent.columns === 1) {
                            break;
                        }
                    }
*/

Screenshot of the code attached.