Mine were stuck on with RTV when I got the car. I "repaired" them with small magnets, 5 stuck on wheel 5 on trim. I used Sikaflex to stick them on. Do the 5 on the wheel first (check to find best location for both wheel and trim) hold them in place with tape until the Sikaflex sticks. Let the tape stop any sikaflex being higher than the magnet face. Remove tape and cover with magnets with a thin sheet of polyhene from a plastic bag and allow the second set of 5 magnets to magnetically "stick" to the first set holding the plastic in place. The plstic stops the Sikaflex on the trim stickingto the wheel. Apply a small blob of Sikaflex to each magnet and another larger blob to the corresponding location on the trim. Don't forget the trim has a locating pinso omly goes in one position. Place the trim in place nd again hold in place with tape until Sikaflex sets. You can then remove the plastic sheet. Only disadvantage is ferrous road dirt tends to stick to the trim surface where the magnets are bu is easily removed with a bit of sticky tape.
I'd post photos but am thousands of miles from the Croma in a land bereft of Fiats (I have seen two Alfa's though).
The only reason I know why these wheel centres get broken/damaged is due to fitter incompetence and/or time pressure. You can not just bang them in. You have to carefully offer the cap to the wheel nave recess, square it up over 360 degrees and then and only then whilst supporting the square alignment tap them in a single measured strike. If the strike is too soft nothing happens. Try again and when right the cap will just snap into place.
I liken the fitting of the caps to that of closing the bonnet. Lift an inch to two and dropped. If it does not latch closed try again from a slightly higher height.
Enter the monkey who drops from a hight height or who presses home leaving small or large depressions in the bonnet.