storyteller
Veteran
Ichigo ran after the lure of the Grand Fisher, and she went to retrieve him. She could have easily been killed while off guard then.
Also, remember, Isshin didn't get his powers back until probably between the beginning of the series and right before the Soul Society Arc. He wasn't able to do anything. And if Masaki WAS a Quincy, who knows what the state of her powers were.
Even a teenage quincy was detecting Hollows without a problem wherever they showed up. Masaki getting caught off guard seems completely out there to me and even the fact that she brought Ichigo near a powerful hollow seems like a reach for any Quincy. His pops regrets that he didn't get there in time (pretty sure he said this to Ichigo on the anniversary of her death) which means he thinks he coulda done something.