Carlo
A lot of the pipe connections are not suitable for jubilee clips, the pipes have a metal end/insert which fits into the housing, then a big circlip goes over the top to hold them together.
Hard to explain, but there is no easy way a jubilee clip could be used on these joints.
MarkyP's mechanic is right.
The metal "insert" or "end" on the pipe has two small lugs, which the circlip holds onto to stop the pipes coming apart. However these lugs are quite soft, and can wear away or get damaged, if that's the case then the pipe needs replacing.
First thing to try is buy new circlips (careful to get the right size, there are several, and ETKA listed some wrong part numbers when I needed some for my
Ibiza - best just buy a mixture of sizes they're not expensive) - and before fitting them, make sure the pipe joints are pushed COMPLETELY together. If they are not, the circlips will sit "on top" of the metal lugs rather than behind them - which means they won't grip, and can also damage/wear the lugs.
This is probably all hard to understand if you haven't had the joints apart yourself to see them...