Enzootic abortion can cause weak live lambs as well.
It sounds like it could also be meconium staining of the lambs due to a prolonged, tricky or stressful birth.
Or it could be that ewe had an isolated condition/trauma/infection that stressed the lambs before birth.
If you have any others that are concerning you, it would definitely be worth asking the vet to take a look.
They can sample lambs/placentas to check for abortion agents, but at best only 50% of abortions get a diagnosis.
As the lambs are alive, I'd wait and see, if you have any more weak, early, dead or that look odd colours/placentas, you could test them and/or blood sample your ewes later in the year for enzootic abortion (EAE) (your vet can advise you how and when to test the ewes).