It's not their fault - it's due to lack of planning ahead on your behalf.
Other people don't know your timetable unless you share it with them and communicate what it is you need and by when. They can then plan their time accordingly or at least you've set expectations.
With holidays, half-terms and long weekends constantly adding to the planning mix - you need to be aware of the impact of other people's timetables on your scheduling and completion of tasks.
Give them enough notice. Let them know what you need and when. Don't communicate this two days before you need it!
Follow-up - are they on track? Will you get the information you need when expected? While it might not entirely be your fault - some team members may not be that good at planning themselves and over commit or just simply forget, if you follow-up in a timely way it will act as a reminder.
Get the important stuff done first - not all tasks relating to a project are equal. If you have a looming deadline, what's the really important stuff that needs to be done. Don't waste time on unimportant tasks.
Get them in order - some tasks need to be completed before others can start or continue. Don't get caught out by starting work on a task and then realising there are additional tasks that need to be completed first.
Planning Ahead (Photo credit: Chiot's Run)