We got 2 ducks and a drake about a year ago. They were very shy and frightened in the beginning and they would not leave their pen...but that has changed dramatically after a a couple of months
What I found is that ducks are very much a creature of habit...what they do for a few days is what they will keep doing. And I guess they feel save in their house or pen. I find that ducks take much longer to settle into a place than chickens! But they do a great job on the slugs
It is a bit like programming them... If you want to change where they spend the day you will have to lock them out of the coop or pen. I had to keep pushing the ducks back into the garden. In the beginning I had to do that several times a day because they kept going back to the pen and sat infront of the locked door. After doing that for a few days they would eventually spend more time in the garden. I also offer them water where I want them to hang out. A kiddies pool does a great job and is like a magnet for them.
I also feed them in the garden in the morning. I throw some food on the ground for them andthen let them out of their coop. They run out of the pen for their food and spend most of the day in the garden! It took a good few months though to establish this routine...