Vertical layout – possible css issues

Yes, this is a CSS issue. You have created 3 concurrent items on the Monday, resulting in 3 columns to display that day, and there was no style defined to handle this. To fix it, go to the Stylesheet editor section for Weekly Schedule and add:

.verticalcolumn3 {
    float: left;
    width: 50px;
}

Now, even with this, you will still have display issues since the content div of your site is not very large. You can see to make it larger or make columns thinner.

Something else to consider is that if the events that are at the same time are unrelated, you could actually just created multiple different schedules using the plugin.

Let me know if this helps and please consider rating or donating to support the ongoing development of this plugin.