How do you go about asking for retainer for a project you’ve committed to, but hasn’t started yet?
Here’s a scenario:
Someone is putting in a bid on a project and wants to include you as one of the writers. You say yes, knowing that your part of the project won’t start for another six months.
When your project is supposed to start, it gets delayed for a month.
When that month passes, the project gets delayed for another 3-4 weeks.
In the meantime, others have asked you to do work and you’ve said no, knowing the first project would be starting very soon.

What do you do in this situation?
Do you ask for a retainer?
Do you not commit until the project is ready to go?
If you don’t do that, how can you have enough time to work on that project?

Normally I find that my projects start within one to two weeks of when they’re supposed to start. However, with some clients, they habitually delay the project and I’m left w/o anything to do. I know this is partly my fault, but any advice?