Are you take the lens out, then look from the rear elemen thru to front element of the lens?
If you found something inside the lens housing, then it maybe the dust because using canned air to clean dust off lenses, and the canned air can sometimes carry so much force that it can drive dust particles inside the lens. Or maybe the mold if you are in the tropical climate area.
If you look thru the camera, it maybe something on the mirror, then you can use the hand blower to blow it. ( I only use the hand blower and the soft microfiber cloth to clean)