2 yrs ago I had my 100 yr old hardwood floors sanded , etc.
To speed things up, rater than coating with poly, they applied some kind of varnish or shellac.
I dust regularly,and just use a barely damp cloth with plain water for wet cleaning.
HOWEVER, now I notice that the floors are losing their luster and shine.
Is there any solution to bring this back? I read in general, that waxes, etc leave a film after a while.
Is there a fix for this ?
Thanks for any suggestions.

Just two years? do you remember who it was that re-finished the floors? They may have suggestions. A modern finish can be re-coated time and time again, so the floors will not need to be sanded again, if wax was not involved. If the floors were waxed, after the seal coat, you will need to re-wax. Usually a cleaning schedule will be recommended by the company who finished the floor.
Sometimes the cleaning products leave a residue that will dull the finish. A hardwood floor and laminate cleaner will dry without a residue. With a film type finish, you should be using a product that will help dissolve the dirt, but will not leave a residue, if it isn't completely rinsed off.