In an apparent effort to prevent exploitation, and maybe also to conserve server resources, LOTRO has a rate limit on how many instances can be launched within a given period of time. As of Update 31 the official limit is 15 per hour, 100 instances total per day. When the limit is hit, the player gets an alert, which appears as a lock symbol, which when hovered over, shows the countdown timer for when an instance can next be launched.
Additionally, text will appear in the "Error box" UI element, if attempting to launch an instance via an NPC, while instances are locked.
The same error text will appear when hovering over the "Launch Now" button of Instance Finder, and all of the instances listed therein will be greyed out.