As Bhavya mentioned you might be getting tricked by the way multi-sort works. You can sort on multiple fields in Discover now. The first one you click on (or the one that is already selected in timestamp's case) will be your primary sort. The next one you click on will be your secondary sort. If you don't want to sort on timestamp, just click the arrow in the timestamp column again to remove that sort. There are tooltips on the arrow buttons to help you understand what each click will do.
About the fields without sort buttons: these are probably not sortable due to how they're mapped. text fields are not sortable by default. If you have a keyword version of that text field then you can sort on that. The keyword fields usually don't show up by default in the sidebar because they're typically multi fields that don't exist in the source document. You can make them visible in the sidebar (so they can be added as columns) by opening the settings in the sidebar and unchecking "Hide missing fields".
