Can an employer with hold info and prevent you from filing a grievance?

Hi, i work as a forklift driver for a warehouse with a union. I am a young female, ever since i got the forklift position i have had managers who were once really nice to me, start to act differently. Got write ups for no reason. Ive been given issue when i got sick, as if i did it on purpose. Or when i hurt myself, they acted as if i lied, or mocked me. Today i had my clipboard stolen from my lift when i was on break. Now this is my personal clipboard that i paid out of pocket for, it had signed request off slips on it, my volume order paperwork, and merchandise drop list for the day, as well as a picture of my deceased cat on it. Which i use to help subdue my anxiety till im able to get home. I asked around all day and looked everywhere for it couldnt find it. Later when the warehouse manager came in my supervisor, who i am close to, asked to look at the security camera footage to see who took it so they could find it. I recieved my clip board after opening, but everyone refuses to tell me who took it, why, or where it was hidden. Meanwhile i had to deal with anxiety attacks throughout my shift. I wanted to file a grievance against who ever stole my stuff but management refusing to tell me who the person is, which is preventing me from doing so. So my question is, are they allowed to do that? Are they allowed to protect whoever is attempting to bully/harass me? They claim they "talked" to the person and it's "being handled". But they said the same thing when my little sister was being harassed, and nothing was done. And management went around calling her a liar.