Since this is a mainly english lanuage newsgroup I'll try to translate
this post, for I have quite the same problem with my 1995 Punto 1.2:

<BART>
The problem (1994 Punto diesel) is, that when trying to fuel the car at
nearly all gas stations, the fueling pistol immeadiatly closes shut
after triggering. This makes fuling a drag, since you have to get the
fuel in nearly droplet by droplet.
There are gas stations with slightly smaller pistols, with which the
fueling goes pretty well, but in about 9 out of 10 cases the process
doesn't work.
Bart now assumes that the openinmg for the tankpistol at his car is too
small and asks for someone who might have some advice what to do.
</BART>

In my case, pulling out the pistol some centimeters before triggereing
works in most cases. The problem with that is that you have to keep
holding the pistol during the fuling process for it's not in deep enough
to lock.

Any other ideas or comments?

Greets

Eric