The IRS's priority is to get paid. This is why the IRS is often so willing to reach a settlement with a delinquent taxpayer. The same is often true when it comes to individual state taxing authorities. As a result, simply expressing a willingness to pay back what you owe is often enough to reach an agreement that keeps some of the worst IRS penalties at bay.
One of the big reasons for being refused an IRS installment agreement is a failure to be current with tax filings. Unfiled taxes can prevent the IRS from granting you an IRS payment plan. Simply filing late taxes before submitting an application for a new installment agreement is often enough to be approved. The goal of the IRS is always going to be getting you into compliance to allow for as much money to be collected as quickly as possible. Getting into compliance and making sure that all outstanding factors that could be preventing your eligibility are settled will be important if you're at a standstill with negotiations.