Consultants may not be quite as excited about your Request For Proposal as you are. If you really want someone to bid on your project, you may want to let them know that you are interested in them. Like the nonprofit organizations they work for, nonprofit consultants operate on a slim margin that does not allow them to waste time and therefore money.
Proposals take a lot of time to write. Consultants will hesitate to take the time to write a proposal unless they believe the process to be truly fair and open. They will also hesitate to waste time guessing on what you want if you have not been clear.