Unionizing is difficult in jobs that have such rapid turn over. A petition to the NLRB can take more than a year to process, at which point half your coworkers could be gone. It's also difficult to petition to include part-time worker. I don't know if those have been your problems, but they've been mine when I've organized. It's hard to organize people who believe they can simply get a better job if they quit.

This is all by design by the way. The NLRB isn't supposed to work.