The closer you are in strength, the more uncertain it becomes that you will be able to shield someone. If you are exactly equal, the chances of success are very slim. In fact, in Nynaeve's case, it was proven that although she was equal to Talaan, she was unable to shield her, and was in fact shielded herself because Talaan was more skilled.

So it became a battle of skill, which Nynaeve lost.

As for Verin and Graendal's cases. See above. Verin and Alanna's shield was hopelessly weaker than Rand's connection to the source. So much weaker that he did not even have to bother to resist it.

Same with Graendal and Verin's case. Graendal was channeling through an angreal and the thickness of her connection was either exceeded Verin's shield strength by too great a margin, or Verin's shield was of insufficient precision to slice through it.

We know Lanfear is very dexterous, and can therefore no doubt create shield with a much sharper "edge", meaning in effect that she can punch above her weight in strength.

But as I said before, if it was Lews Therin in front of her, instead of the untrained, ignorant Rand, she would have had no chance.

Ishamael could not even cut Lews Therin off from the source. How on earth could Lanfear hope to do it then?