There have been numerous spottings of debris and "slicks" which have turned out to be something other than what is being searched for.
Is this area of ocean relatively littered with debris?
If so, and if the aircraft left little in the way of large floating pieces (this includes any option of the aircraft being outside the search area), then rather than looking for a needle in a haystack, the searchers might be trying to find a needle in a field of other needles.