Amys is stronger than Melaine, but Melaine is still a very strong Aes Sedai. So if Egwene is as strong as Amys and Melaine combined, then Amys is more than half Egwene's strength, but not much more.

And you probably only have to be around half someone's strength to be on the borderline of being able to shield them.

The question is, how strong is Barasine? If she is as strong as Amys, then she should be able to hold Egwene for as long as Amys can. Which is certainly not a secure way of holding someone prisoner.

If she is weaker than Amys, then she cannot hold Egwene.

When Katarine asks why more than one woman was needed to hold Egwene (and Egwene made clear she meant Barasine should hold Egwene), Silviana doesn't reply that Barasine would find it hard. Even Egwene doesn't think that. All she says is that Egwene is tricky, so to be safe, she's adding one more.

By my count, Egwene is around 70, and Katarine and Silviana will be around 50, with Barasine being in the 40s. Which makes it possible for all of them to hold her with ease, which is what Egwene thinks too.

As for why Amys struggled... maybe it is a skill thing? Reds pretty much live to shield, and maybe the Wise One's have cruder shields.