Just remember that there really aren't "marks". Timings are created just like effects which means they have a duration. Every timing effect has a starting edge and an ending edge. So you can have a timing effect that goes from 1 to 2 seconds and then another from 3 to 4 seconds with nothing in between 2 and 3 seconds. If you want one in there just select a range and hit "t" then drag the edges over to connect to the existing timing effects.
