It could be that the carto has gotten gunked up a bit with
juice. Juices that are dark in color and/or high in VG tend to shorten the lifespan of cartos. I love to
vape VG juices but find that they are better w/ atomizers than cartos (others may have luck w/ that is just my rather limited experience here.)
Cartos do have a limited life span and it may need to be cleaned properly to use it again. What I would try is to take carto off of the battery, wipe the battery end and blow out some of the
juice into a paper towel, wipe the contact thoroughly to see if anything is impeding the air flow and reattach. If this doesn't work try a new carto. There are a number of methods for cleaning cartos that you can try to breathe some new life into it. I've boiled mine (try utube for some good videos) and allowed them to dry with good results.
Good Luck! Hope that this helped.