Generally, If you wish to service the reservation directly (by calling the cruise line) they will refer you back to the TA you are working with. Which means attempting to contact your TA and waiting for a reply. They add a "middle man" that must first understand your request then relay that request to the cruise line. Some don't participate in the upgrade bidding process and some charge change fees not related to the line you are sailing. Some have great TAs that are responsive and actively monitor for price drops and such but I have found that I must make them aware of pricing changes.
I'm also jaded. Way back when internet based cruise travel agents were a new thing one of the largest abruptly went out of business. As I recall, the son of the business creator took over and mismanaged customer deposits. They collected payment but did not forward them to the cruise lines. I got very lucky when Princess alerted us to the lack of payment and were able to resolve it, many others simply lost their payment and cruise.