Perhaps you could save the unfinished weavings, continue with your next lesson
and have them finish them up when they have extra time. Let them know that it
is part of their grade and they need to complete it in order to pass.
Sometimes I will give my classes a Free Art Day/ Catch Up Day. The students
that always complete projects on time get to do an art project of their choice
while the stragglers have an opportunity to finish up all incomplete work.
Most of the time, they finish their work so they can have independent time.