Please don't take this the wrong way, but if the ops are only quitting on you then maybe you need to examine your team leadership style. If it's a universal problem then maybe there is something wrong with the course structure.
Have you tried sitting down and having a one on one conversation with one or more of the ops who have quit.
Yeah not to insult you in any way... Heck I don't know you... but it sounds like there is either a problem in the program as a whole that makes it acceptable to quit or there is a problem with your leadership skills. If it's a program wide problem, like Avkid said, you should be talking to the department about giving you some support. If you quit in the real world you might never work again.. and because of that it shouldn't be acceptable in educational theater either. If it's something you personally are doing (or not doing) you should know now so that you can work on it before it damages your career. Logos has a great idea of talking to someone who has quit and asking them honestly to share why.
Quiting has been discussed from time to time in threads. As far as I'm concerned the only time it is ever acceptable to quit is if your feel the theater/performance is so unsafe that you, the cast, crew, or audience is in SERIOUS danger. And you should only quit after you have made multiple attempts to make the show/space safer. You also need to know that it may take a long time for your career to recover from this decision.
im not expert on this, so take this only as my 10 cents
but i would check in with your crew every so on to make sure their ok, even outside of their theatre workings, if they don't want to talk don't push them but if you start to see them slipping into unhappyness speak to them, make sure you don't keep losing them
we have never lost a tech, yet
and our lighting op's work on an LSC axiom 36
or a lanbox
I agree that the only reason shows should be withdrawn from are because of severe unresolved safety concerns, severe illness or death of a family member. Once you commit to a job, you should stick to it.
Obviously in a more amateur environment, people see these unwritten rules as optional, and walk if they don't like the director or SM, or because they'd prefer to hang with friends or party on particular nights. These people just have to be remembered as less committed individuals, and in my case I will not ask them to help again.
As mentioned by other posters, if you have a constant issue with people walking from you, and you only, you might want to revise the way you treat them in the working environment. You may be expecting them to do too much, treating them unfairly, etc. Although this isn't a reason that they should walk for (instead talking it over, fixing things, or just living with it and noting down not to work with you next time), it's going to happen whether you like it or not.