Really stupid question about door knobs

[SchnitzelVonKrumm]

All doorknobs replaced when house was being refurbished last year (Victorian-style, porcelain, like this. Three of them keep falling off or slipping so the mortise doesn't engage, the rest are fine. Any idea what the problem could be? I think those three doors are thicker than the others (suspect they're the original Victorian doors), could that be it?

[PigletJohn]

Have they got a screw underneath to grip the spindle? Or to screw into a hole in the spindle? If the spindle is fully pushed into one knob, there will be less spare for the other.

Compare the spindle length to the thickness of the door. Internal doors are usually about 35mm thick, or 44mm in more grand houses.

You can buy spindles in various lengths. www.ironmongerydirect.co.uk/search?SearchText=spindle

Are they rimlocks?

[SchnitzelVonKrumm]

Ours are about 50mm!

[PigletJohn]

thick paint?

[SchnitzelVonKrumm]

Mortice. Yes, could be paint I guess.