I have replied to people in the past on Forum re this subject but I can't find the thread.
Basically I have worked on many large vans including Fiat Ducatos, Iveco Daily's and others.
They are quite large doors and need to be kept well oiled and the runners free of debris, due to their weight they go out of line easily the bottom roller arrangement can be slackened and levered back into place from memory three 13mm bolts, when slide towards the shut position the metal trumpets on the end of the door need to align correctly with the plastic spring loaded buffers as the door pulls in close to shutting also the top lock mechanism on the back end needs to be aligned correctly and again well oiled.
If you push the door almost closed you should be able to see how it is aligning and adjust to suit.
When all working well I try to always pull on the handle to close and at the same time push at the rear of the door to take some of the loading off it.
Often drivers just drag it across and slam it in to place which only works for so long.
Generally the parts I have replaced most often have been the top lock catches damaged through misalignment and the plastic spring loaded buffer pieces.
