Unfortunately this can happen Nat.
On my first round I had 15 eggs and on day 5 only had 2 morulas which we transferred back and none to freeze. Ours slowed down in growth on day 4 too. Its so frustrating I know. We had 20 this time and 15 fertilized. We had 11 frozen on day 3 which looked really good. So I'm hoping when we thaw them out to day 5 that we don't have same issue. Did you have a blastocyst ET today?