No, more could not have been done by FBI to prevent the Orlando attack. Anyone with a gun can do something like what happened in Orlando. The reason that this country labels it terrorism is that the country does not want to admit that it needs to get rid of guns. It does not make sense to believe that allowing people to own certain types of guns is a good idea.
No, I do not think that the FBI could have done more to prevent the Orlando attack because there are over 800 individuals on the terror watch list and the FBI doesn't have the personnel to watch all these people. In addition, it's very easy for a mentally disturbed individual to get a firearm in the United States. I think the FBI is doing all they can to stop these violent attacks, and could not have done more to prevent this attack.
According to media reports, this man was investigated at least twice prior to the shooting. Both times, no evidence was found to show that the man was a danger to society. It's easy for us to look back and say that more should have been done, but if the FBI found no evidence of terrorist ties, then there was no more for them to do but keep an eye on him.
Sometimes things can be prevented, sometimes things can't. I agree that the FBI had information on the man responsible for the Orlando attack, and that they were keeping an eye on him because they suspected he may end up doing something, but at the end of the day it was just speculation. You can't arrest and detail everyone on "just incase" information.