I don't know what is normal vocalization for Ancona ducks, but one of my ducks also only quacks when she is upset/startled/desperately needs me. She probably only quacks once or twice a day, and sometimes not at all. Compared to my other ducks who are shout quacking all day! Otherwise my quiet duck makes a modified peeping/clucking noise. I think it is adorable. She's almost 7 and she's always been like this. She's really submissive, I assume that has something to do with it.
I had two pekins that didnt make any noise unless they were excited about eating or were freaking out when they didnt want me to leave their sight. They didnt make any "just being alive as a duck" noises throughout their day. Their "here comes mom and that probably means treats/attention" noise was also not a full quack, it was a sort of basey/chesty chirping. Their full quacks were calls/panic shouts when they couldnt find me (or when I picked them up, because at that point I turned into the devil and I was definitely going to kill them despite that never having happened any of the hundreds of times I picked them up in the past)
Some ducks are just quiet (lucky). If she doesnt appear to be in any other kind of distress, sick, etc then I'd say shes fine, just shy.