There could be a couple of reasons for this
1. The quote has closed or expired.
Every quote has an expiry deadline that you set when you email the suppliers. If this has passed the quote will close and suppliers will no longer be able to submit prices. To re-open the quote go to the job bag and send the 'Quotation Reminder' email. Find out how to do that on the Costs screen.
2. The job may be cancelled or closed.
When a job is ticked as cancelled or closed, any outstanding quote requests will be cancelled. You can reopen a job as long as it has not been charged or invoiced. To check go to the Main Details tab of the job bag, at the bottom of the screen you will be able to see the status of the job. If the job is 'Completed' or 'Cancelled', untick the box and 'Update' the job. You will then need to resend a quote reminder to suppliers, at which point suppliers will be able to see and submit a quote request.