Employer refuses to apply vacation time to when I was sent home due to over staffing, even though they advocate for use during slow periods. Should I email them to remind them for a 3rd time that I’d like to use it?

I’ve been averaging about 15 hours a week, and things have been rough. I got sent home twice along with several other people for over staffing reasons and signed up to receive vacation time on those days. Employer did not apply them to my timesheet. I met with them in the middle of the week to remind them, and they said “of course,” blah blah blah. The negligence is so irritating. This is the 3rd or 4th time they said they would apply my vacation time and didn’t. Should I email them to remind them yet again? Or should I just leave them alone and suffer through the hardship? The last time I sent them an email they acted very annoyed.