Every alchemist excelled at some fields of alchemy. Of course there was a filed was they just plain sucked at.
Although Ed was considering a genius at alchemy, no one actually knew his weakness. Perhaps Ed was just good at all forms of alchemy.
Roy knew differently. Being together for a half of a year, you learned a lot about each other. Not on purpose, it just came naturally.
It happen on accident, that Ed didn't do well with fire alchemy. True when he was focus he did a decent job.
However when Ed was distracted, he sucked at it.