Are you experienced at applying raddle? It's surprisingly easy to put it on in such a way that the ram doesn't mark the ewes.... Re-read your opening post and crossed that idea out!
Otherwise, the only other things I can think of are congenital or other infertility (ie., arising before they came to you) or mineral deficiencies. But in both cases, it would be extremely unlikely that all 5 would be so profoundly affected.
One thing I have definitely seen before is a ewe getting tupped and then lambing to a date 17 days earlier
So to me, the most likely improbabilities to consider are :
- they did get in lamb before they came to you (for instance, if one of these ewes, or the ewes they ran with before you got them, is actually a tup, or a wether that isn't fully wethered - and I
have seen both misgendering and incomplete wethering happen to farmers with decades of experience before, though never both together.)
- this tup has fertilised them but for some reason the raddling didn't work
- one of your castrates isn't fully wethered
- there is a humungous fertility problem on the farm they came from