You can try talking to staffing, and explain your absences.
You only get terminated after getting 3 consecutive letters, so your good for now. The acceptance rate use to be 50%, now they average the acceptance rate from the list you work on so it changes every time they evaluate employees.
You can also grieve your letter, but the union will not take any actions until you get fired, (some ****ing stupid clause in our agreement article 9.106)
If you do lose your job, union will proceed with the grievance and it ends up in arbitration, it usually takes 3-6 months to get your job back, and you do not get compensated.
union does not give a **** about temps, I learned this with my experience above, if they truly did they would negotiate what the **** "reasonable availability" means and have it put in writing with a set percentage or so.