Try cutting the ear hair to reduce the weight on their ears especially if they are puppies.
Second, sometimes puppies ears go down when they are teething.
If you are concerned with their ears staying down. take some 2" medical tape and tape them up. Its not harmful or cruel. I had to tape Chewy's ears when he was a puppy because they would flop over when he was teething. Both ears stand up but occasionaly one will down if there is a lot of hair. Here are a couple websites to show you how to do it. Hope this helps.
Top Taping Yorkie Ears Taping Yorkshire Terrier Ears Stand Erect Yorkie Ears Jon-An's Yorkshire Terriers (How to train Yorkies ears to stand)