PTO usage

Why is it so hard to get PTO approved?

For the past 6 months, I've been working 6hrs and 6 days a week and finally decided I wanted to use my PTO because my body is starting to hurt a bit. But every time I ask my full time supervisor, he says there's already too many people on vacation or this month is already full. But then I show up to work and everybody is there. I feel like they're not approving it because after a couple of hours in unload, they push me to preload to load some trucks because we are severely understaffed and our new hires are quitting fast.

Currently in a SoCal hub