Mine were similar - it's just a bad design - you can clip them back on, but they'll ping off again
I went to Homebase & got some mega-strong double sided tape & some Araldite 2-pack glue and did the following:
Offer the sills up to the car, to get them more-or-less in the right position. I used some insulating tape, and put a strip of this on the car body ABOVE where the sill top line would be.
Then I put the double sided tape about 1cm below this, peeled off the backing, and pressed the sill up to it (was enough to hold it in place).
Then I mixed up the Araldite, and syringed it in on top of the double-sided tape all along the length of the sill.
Next, I got a bit of 1x1 timber, and some long g-clamps, and clamped the wood against the sill (doors opened, with the other end of the g-clamp effectively inside the sill. Clamped it all tight until the araldite was beginning to get squeezed out. Cleared up the excess Araldite with a rag, removed the insulating tape strip, and went away for an hour. Came back & released the clamps, and the sill is SOLID! Stuck fast & going nowhere.
Hope this helps.