For me, the problem seemed to be that I didn't have enough credits to cover the cost of the costume change, even though the cost was clearly less than the amount of credits I had.
Let me explain: I had a costume change that cost 110 EC, and I had 130 EC. After making my changes in the Costume Editor, the cost correctly showed as 110 EC. I pressed Modify, and nothing happened; I remained in my old uniform.
However, after I sold some stuff and got up to 1000 EC, the Costume change went through without a hitch. And only 110 EC was subtracted, even though I needed a greater amount before the change would go through.
My guess is that, under the hood, the Costume transaction code tests against a higher EC value than what's displayed, even though it ends up subtracting the right amount in the end. Definitely a subtle bug, if it is one.
I recently bought TOS and Deep Space 9 uniforms from the CC store last week. I have had problems changing the uniforms. It won't let me change into these uniforms. It has been frustrating to say the least. When I go to the tailor on Earth space dock, there still is a lock symbol on the drop down menu for both TOS and deep space 9 uniforms. How do I unlock them? I went back to the CC store and it was confirmed that I bought these uniform options. I've spent a lot of time trying to change uniforms, but my character while playing has the same starter uniform. The funny thing is, the uniform changes I can make appear on the character selection menu. Could you send me step by step instructions on how to change the uniforms? Why is this process so complicated compared to other MMO's? It seems to me that a player should be able to change it on the inventory menu.
P.S. I read a lot of similar complaints online from other players. Perhaps STO should stop charging for these options, until they work out the bugs!? Otherwise, I think STO will lose a lot of players when The OLD REPUBLIC MMO comes out.