all the struts were replaced and thumping continues with mechanics unable to fix. What is the main problem in this situation?

 

The location of the thumping? Rear, front, which side?

 

Look into all rubber parts/bushings. Also look into the sway bar links/stabilizer bars. My drivers side link just blew out it's bearing and it clunks when the front of the car dives when stopping. Just ordered it before typing this.

 

rear thumping. passenger side. maybe the sound travels along the undercarriage. Did the rubber parts including sway bar rubber. If the wrong struts were installed would that cause the problem? It has been persistent for six months or more. Unable to contain it as it returns after a few weeks.

 

Sounds like sway bar bushings. Start there as they are simple to replace and cheap.

 

I was hearing thumping in the rear passenger area & bouncing all over place . Today, installed some rear Monroe OESpectrum Refex struts with the OEM Mounts that were on the RX.......
No more thumping, no more bouncing . Decent struts, Caddy type ride, Lifetime Warranty, can't complain. Can't compare to OEM struts since mine were finished with most likely 140,000 miles of use.
That was in my case for the thumping issue. What brand of struts did you install?

 

Did you install strut tower bushings as well? On our Subaru there are rubber bushings in the top of the struts. They are a separate purchase from the strut inserts. They will cause clunking even with new struts if not replaced as well. Also sway bar end links have bushings too as well as the sway bar itself.