Some puppies can get very picky from time to time, and if you try to cater to their whims you'll find you just can't keep up!
One of my Rottweiler males was a horribly picky eater as a puppy, and like you I thought that was odd for such a big, strong looking dog. Thing is he'd been spoiled as a tiny pup by getting fresh minced beef and milk straight from the cow (literally, he was born on a dairy farm)
Needless to say when he encountered dry dog food he was less than impressed. I tried to encourage him to eat by doing pretty much what you're doing now. I tried several different dog foods, and then graduated to actually cooking him things like scrambled eggs, with cheese and fresh beef for breakfast ..... what on earth was I thinking
It didn't matter what I did, after a couple of days he'd just start picking at whatever food was flavor of the day, and I'd be back looking for something else to try.
When my kids started to complain that I cooked more for the puppy than I did for them I realized that it was getting out of hand. So I quit, cold turkey.
I went out and bought him a high quality, large breed puppy food, and filled his dish. That was it. Of course, he didn't hardly eat anything for days, but eventually he realized that he'd either have to eat this or be hungry. And guess what? He ate it, and lots of it. From that day, we've never looked back.
I'd been told that dogs won't starve themselves, and that they'll eat when they get hungry enough, but I was afraid to put it to the test, until I reached the end of my rope.
Well, I can tell you.... every word is true. Pandering to their pickiness, just encourages it and creates feeding issues and problems just as it does with babies and kids.
As proof, I can now tell you that the puppy in question is a 120lb adult, who looks as though he's never missed a meal in his life! Don't get me wrong, he's not overweight (that's VERY bad for Rotties) but he's at the top end of the height requirements in the breed standard, and he's a HUGE dog.
I hope this huge essay has helped you take the plunge and have the confidence to choose a good food (that's your responsibility) and then let your puppy choose to eat it -or not- (that's his responsibility).
best of luck!