Michael,
Indeed many of the earlier missions, both those supplied with the sim, and those created by others, are flawed.
Some have been made so by changes in the sim, such as the inability to cast off static ships that are moored when the mission begins.
Others can be completed only if the player can read the mind of the creator. Many of these involve hidden waypoints—always a potential problem—that the creator has not ensured that the player will hit.
The system of having missions submitted to, and vetted by, Custom Mission Managers is intended to reduce the number of faulty missions.
An understanding of how missions are constructed can be helpful in playing them—even if you are not ready to construct them yourself. There is a tutorial in PDF form that you can download from FAQ and Announcements > Tutorials.
If you have difficulty with a mission, and can’t find help on it posted somewhere on the Missions board, just move on to another one. There are many—of varying quality and difficulty.
Speaking for the CMMs, we always want to hear about any difficulties you encounter with a mission. We ask that you identify the mission by its exact title, and give as detailed a description of the problem as possible.
Regards,
Marty