I put in Saerin because she was trained by the Daughters of Silence. Perhaps there was a block, which prevented her from reaching full potential. But...

Several ways to parse this. Is this mention of Elayne a continuation on your previous sentence "Why would a sister not reach her full strength, with all this implies for an AS?"

In that case, I can think that this Aes Sedai holds a high position which kind of nullifies the effects of her not reaching her potential. That's still not a reason though.

Another reason to mention Elayne, and not Aviendha (who's at least very close to Elayne's strength since Elayne had no trouble handing over her angreal to Aviendha during the Bowl of the Winds episode) is of course Elayne's pregnancy. Terez has said Elayne hasn't reached her potential by aMoL. She even said "not even close", though whether she means 60 or 70 or 80% of her strength is still up for debate.

It could, of course, be that pregnancy prevents you from ever reaching your potential, but I can't see that as being true. Could it instead be about Elayne being unable to access the Source successfully all the time? Maybe she hasn't come as close to her potential as she could because she fails to reach the Source so often.

So, maybe an Aes Sedai who fails to touch the Source very often for a significant amount of time. Maybe someone with a block that causes this?

This is proving to be quite the interesting mystery.