ducks never leaving coop - not settling? help!

We got ducks about 3 weeks ago. We got 4 khaki campbell hens, POL, about 16 weeks old, from a very reputable, small breeder in our area. They live in a house used by previous owners for ducks and chickens, it's a metal shed with a concrete floor, scattered with about 6 inches of straw. The run is about 3x5 meters or so, has a concrete area right outside the house, sloping gently to grass and weeds, a sunken bathtub pond, and a few corners to hide in, as well as a compost bin with lots of worms.
We brought them home in boxes, it was about 30 min journey, not too hot, let them out and they ran into the house, they were very scared and didnt eat any food for a day. We have layers pellets (nonmedicated) and cracked grain for them. On day two we discovered that if we put the feeder inside the house they would eat so we did that although I dont want to continue doing so because it is messy and I'm worried about attracting rats. But I figured until they settled in, ok, we'll do it that way. So they were eating, but only if the feeder was in the house. Even if it was on the threshold, they would not eat. They drink plenty of water.
After 1 week, we got out first egg, so I thought they were settling in. Then that afternoon I went to put them to bed for the evening and found two of them drowned in the bathtub We got two more ducks from the same breeder, who said a lot of people have lost birds (we've had really unusually cold weather) and it wasn't our fault. That was nearly two weeks ago.
We are still in the same situation with the ducks never wanting to leave their house or eat unless inside. The two that drowned, that was their first voluntary outing, and they died so I guess the others are very wary? But I am beginning to worry that something is wrong with their environment that they are not settling down properly. We are getting one egg a day, I don't think it's always from the same duck as they are different sizes (the eggs I mean) but I can't be sure. But only one. They will eat, but only if they feeder is inside. They pretty much spend all day standing around in their house, they don't go out to forage, eat greens, eat worms, or anything (they can't splash in the tub at the moment anyway as it's frozen solid).
We were planning on getting 1 drake, an older duck about 1 year old. Do you think having a drake would help and make them braver? Or would he bully them? Is there something else I should be looking at?
We have foxes around, but there isn't any evidence of attempted invasions and we haven't seen or heard immediate evidence. We also know that we have rats - they were in the other half of the shed (it's divided) in the woodpile, poison was put down for them about two months ago and the problem appears gone or greatly reduced, but could they be hassling the ducks at night and upsetting them? A few molehills have also popped up in the duck run - could this be upsetting them? I'm wracking my brains trying to figure out why they are so scared, I just want them to be happy and settle in. Once they are happy with being outside and foraging we want to give them days in the garden to eat slugs! But I can't even imagine them being that brave

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...

Thank you

We got a 9 month old drake who has been free range his whole life and good at foraging, and locked the coop for a couple of weeks, and it seems to be mostly sorted out now. They all go racing out of the coop in the morning and start looking for bugs and stuff. I leave the coop open during the day now but they stay out even in the worst weather. And we're up to three eggs a day, so I guess they're feeling ok.