Where I'm at we don't actually post bid sheets for PT to FT (just for combo jobs). Everyone has to put in a letter each year to get their name on the PT to FT list, then as positions come open they are moved into them in seniority order.
However, when we do have a job that goes up for bid, usually combo jobs, if you don't put your name on the list in the 5 day posting period, you are out of luck. We had a situation not too long ago that an individual was on vacation and did not get their name on the list. The contract here clearly states that if you are on vacation it is your responsibility to have a union steward sign any bids on your behalf. This person greived to get their name on the list and it did not go in their favor, they were not allowed to get on the list.
I would strongly suggest to anyone, especially someone with high seniority to get with their steward to make sure that if they are on vacation when a job is posted they add their name to it.
One thing our HR department does when posting jobs though to help this is to post the jobs on Wednesday that way the posting period is Wednesday to Wednesday that way if someone is on vacation one of the weeks they would still have the opportunity to sign the bid the other week.