I have encounter total read failure with dirty heads, and I have seen where the heads were covered with gunk that took some hard scrubbing to get it off. A head cleaning diskette will suffice in cleaning the heads. Also, you are probably wondering about the compressed air. It is used blow out the floppy drive, just spray inside the drive door and blow that drive out good. Sometimes dust balls or dirt that accumulates inside the drive will obstruct sensors that sense the diskette or whether or not the diskette is write protected.
Some drives are better with dust than others too.
Apple Macintosh Computers have a tendency to not recognize the diskette after it is inserted when they have acquired too much dust. In Mac's the spindle will not even spin the diskette, nothing will happen, and the system will not eject the diskette. Of course, I also have seen older 8 inch disk drives that you could dump a bag of dust in and it wouldn't affect operations. If cleaning the drive doesn't work then you may have a bad drive and it is time to open up the cover. Again, before replacing the drive check a few things first.