selecting messages on message pane

to select two or more messages on the message pane, one method is to hold down the CTRL key and left click on the messages you want to select. This has stopped working on my Thunderbird. When I select a message and then go on to select the next message, the first message I selected is deselected. So I can only select one message for an operation, e.g. "move", "delete", rather than a group of messages.