Drift away from crash site
At this point in time, any debris left floating,will have drifted well away from the actual crash site. Continuing to look for floating debris in the same area, where the crash is suspected to have occurred, will ensure no floating debris is found.
The trouble is, the crash site is close to an ocean gyre. This means the floating debris could have drifted in almost any direction, depending on the exact crash location, and traveled over 1,000 miles by now.
This, assuming that ocean currents and not winds are the main factor in determining the path of the drift.