I’m also working on a project discussing the failure of the first two Agreements of the People. Barbara Taft and Austin Woolrych, who admittedly draws on Taft, both make persuasive arguments. While Taft focuses more on the timing of the second agreement, Woolrych emphasizes the apostacy of the Levellers. I am inclined to argue that the frequent Leveller attempts to divide the army defeated the grandees’ purpose for negotiating with them: to unify the army.
This fits into my larger argument that support of the Levellers within the army was much more widespread than argued by recent historians. Cromwell, Fairfax, and Ireton seemed willing, if unhappy, to go along with debating the first Agreement until Rainsborough hijacked the General Council. Interestingly, the group behind the Case of the Armie was treated with contempt until it was discovered that Sexby (I think it was him, though I’ll need to verify) was involved. Sexby was one the legitimate agitators. To me, this aboutface indicates a desire to reconcile with the more Levellerish section of the army.
In the case of the second Agreement, Leveller recriminations and Lilburne’s A Plea for Common-Right and Freedom, which again threatened the authority of the senior officers, showed the officers for the second time that the Levellers were enemies rather than allies. Why would an Agreement even be seriously considered unless the senior officers believed Levellerism to be a potent force within the ranks, and then why would so many junior officers show such a willingness to defy their seniors during the votes on its articles unless Levellerism was a serious force?
The only real reason I can see for the failure of Agreement is the lack of persistence by men like Ireton and Cromwell, who were in turn most threatened by Leveller attempts to divide the army from its commanders.