Each soft close 'Barrel' is handed - one left - one right. (One is grey - the other white).
Most soft close errors occur because the seat hinges come loose. When this happen, the barrels slightly pull-out of the seat. (There are silver discs on the barrels - These should be tight against the seat).
Once the barrels become loose the first thing that happens is the top cover won't soft close - but the seat still does.
To rectify - puch both barrels tighly into fitting and tighten the hinges. (See below).
To tighen the seat hinges you need to get a flat headed screwdriver underneath the chrome round hinge cover - and 'flick off'. Underneath is a X head screw which id also hexagonal. You can either tighen both by spanner - or screw driver. (sometimes these can be tricky - but it will come off).
If both cover AND seat do not soft close AND the barrel silver discs are firmly located against the seat - the barrel has probably broken.
If so - spare parts can be obtained here: