I signed up for a Nelnet payment agreement, but did not make all the scheduled payments. Will I be eligible for another agreement for the Spring or another term in the future?

No.  If you default or owe a balance on a Nelnet payment plan, you will not be able to sign up for a new payment plan until the first plan is paid in full.   Nelnet reserves the right to refuse approval of payment plans for students with agreements that are pending payments or in default.