Guild Wars 2

Whenever I try and use it to make one piece of armour look like another it won’t let me. Any ideas?

Nope not level 80

You’re not trying to transmute armors from one type (e.g. Light) to another (Heavy)?

I’ve also heard of occasional bugs where the Transmutation window can be finicky in accepting certain combinations of items. Try putting the item whose appearance you want to keep in the Left window, and the item with the stats you want in the Right window.