There could actually be a few reasons for this. The first reason could be the option for the student display option was not switched to "yes" when assigning. If this happened, the lessons were assigned to the modules themselves but simply weren't sent to the "Assignments" tab. The students can still access the lesson by going directly to the subject module it was assigned in.
The second reason is a little trickier. Many times, when a lesson won't show up if you've already tried to assign it, it's because the students have a piece of the lesson (a lesson topic) already assigned. If you try to assign another lesson topic, Moby will still go by the overall lesson and think you're trying to double assign. You either have to let them finish the lesson topic(s) they have from that lesson or unassign them, and then at one time, assign the lesson topics you want them to complete.
In either of these cases, a simple way to get lessons to be reassigned and show up in the "Assignments" tab is to go ahead and unassign them and then reassign. Please see our steps to unassign here and to reassign here.
The third reason this may be happening is if the subject is hidden from your student. If a subject is hidden, it will not allow manually assigned lessons to populate in the "Assignments" section. However, as soon as it is unhidden, your lessons will show.