Being current, and in good-standing on accounts really only rewards your score by the aging of accounts and of reported derogs.
In length of credit, the additional aging of accounts over that period contributes to an increase in age of oldest account and average age of accounts. However, you receive that same benefit even had the account incurred delinquencies during that period. Length of credit history is only 15% of total score, so those increases are kinda slow.
With deletion of that oldest account, your efforts to build length of credit took a big offsetting hit, so any increase in account age resulting from your period of timely payments on other accounts was more than offset by the larger ding due to loss of that one account.
Time also causes the impact of reported delinquencies and derogs to decline, but that again, is relatively slow. A 6-12 month aging of derogs wont build a lot of points. Getting them actually deleted will result in more significant score increase.
With a score in the 600'ish area, there are most likely multiple delinquencies reporting. You state that you have no collections, PRs, etc., but I would venture a guess that you may have one or more charge-offs, which are major derogs on par with a collection, and multiple monthly account delinquencies.
Since you state that you are now current on all accounts, that leaves deletion of old derogs and reduction of utilization as your two main scoring candidates for improvement.
What efforts have you taken to attempt deletion of the old derogs? Have you sent good-will letters to the creditors and/or debt collectors who have reported derogs and you have now paid? That would be my suggested focus.
