are the agents the same version?
I spot-checked a couple and the agent installed on the server is slightly older (7.3.x) than the version running on the app server (7.4.x). Word from on high is that it shouldn't be a problem. Maybe you know differently?
i believe the SUM and some other functions (network deploy too) were introduced specifically w/ 7.4.x, try upgrading one of the agents to 7.4.x and see if that lets it work.
so while most functions will work w/ 7.3.x from a 7.4.x appserver, there are some specific things that may not, i think you're running into that problem.
now... are the servers that are working 7.3 or are they 7.4 ?