I believe you're wrong to think this is an issue related to strength, and not to the personality of the character who is head-strong and has a hard time surrendering. Nynaeve just couldn't do it for a long time, it's not an issue with strength. Nowhere in the books there's an unambiguous mention that a weaker channeler senses the source much less than a stronger one.
We don't know what Morgase is capable of, the Aes Sedai gave up on her very fast, probably right after she did catch the Source. There was no denying that she could channel then, which tied their heir of a very powerful HS to the WT forever, which was all along their point in even trying to awake the ability of Morgase.
The WO did not with Sorilea - they persisted until she learned to embrace the source reliably and could weave, yet according to the notes she might be not much stronger than Morgase (RJ had not decided her strength - he might not have been sure what he truly wanted her to be capable of - so he had a range and a note that she was probably toward the lower end of it). In any case, whatever Sorilea did in Brandon's books can't count, in this light.
But really, it's all about RJ's comment in the notes. He reminded himself that the level below which a channeler can't lift anything with Air (which would be required to make a scarf move) is 57, a way still to 66.
Why did RJ bothered with levels so weak? My guess is that he wanted to go all the way to "0 units", and I bet his mind was also set on getting his 72 levels.