If enabled on your system, you may find that there is a limit to the amount of time available for scheduling timeslots. This usage is computed by adding up all the past timeslots where credit was granted (or participation marked in the case of paid studies) or is in awaiting action status, then adding all timeslots in the future, regardless of credit status. Depending on how your system is configured, the system may only look at all timeslots after a certain date, instead of all timeslots.
You may find that the usage goes down over time, as time progresses and timeslots that were in the future, which had no sign-ups are now in the past and no longer count towards usage (empty timeslots). The timeslot usage and limit is listed whenever you add a timeslot, if usage restrictions apply. It may also be listed when you view your profile, depending on how your system is configured.