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 you may not see a lesson show up under the "Assignments" tab is that the lesson you are trying to manually assign has already been automatically assigned by Moby and is in the student's sequence already. When this happens, Moby sees the lesson is already assigned and keeps it where it is.
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 reassign here. If the lesson is in the "Sequence" tab already, you do have the option to move the lesson to the top. If you do this, it won't send it to the "Assignments" tab, but it will put it at the top of the sequence. A student can now go to the subject itself and this lesson will show up next.
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.