Upgrade of Air tickets

I requested a price to upgrade my air tickets from economy to premium economy, on the 19/05/2015, today is the 25th and I still don't have a answer from them regarding the tickets. I tried to upgrade the tickets before I contacted Travelstart but the airline informed me that they could not alter the tickets, I had to do it though the travel agent I booked them with. Travelstart tells me they are struggling to get though to the airline to request the changes, I find this strange, if I pick the phone up two minutes later I am talking to a consultant who is ready to assist. So why if I can do it does it take them so long? By now the tickets have more than likely been solid out as the airline told me they sell out fast. Bad service from Travelstart, I will just book the tickets directly with the airline the next time I travel and save myself the trouble of dealing with these people.