Sounds like there is a build up of grime left behind by the former occupants. You may have to do a "deep clean" then just maintain with the stick vac and a mop. When doing a "deep clean" there's nothing like a bucket of hot water a good squirt of detergent and a splash of cloudy ammonia. Unfortunately this does entail a hands and knees, scrubbing brush approach coupled with vast and painful amounts of elbow grease. Cloudy ammonia is very fumey, so good ventilation is the key. If you are unable to do it yourself....hire someone. Detergents can leave residue....once you have deep cleaned a splash of methylated spirits in warm water should be enough to keep it sparkling.