Yea it does seem to come down to that Uryu shouldn't of been immune because he should be an impure Quincy since his mother was the maid and yet he was immune. Masaki became an impure because of her run in with the hollow that bit it and Bach was able to take said power without the fear of the hollow poisoning. That means Bach is able to extract the power at it's core and not have it be influenced by any other powers within said person. AKA Masaki having hollow/quincy, the hollow poison might of still been inside her but the quicny wasn't however something to keep in mind is that Masaki didn't die because her power was taken she died because she didn't have the power to fight back the hollow going up against her. Now here's a thought, the hollow wasn't attracted to Masaki but to Ichigo because from what we've learned Ichigo has that hollow inside of him that Masaki was poisoned by.
What we learned from the concept of arrancars is they eat other hollows in order to become stronger and when you consider how strong Ichigo is any hollow would want to feast on the hollow inside Ichigo but Masaki managed to protect him from that happening.
When it comes to Uryu's mother it seemed that her quincy powers allowed her to be healthier but upon losing them she became very sick however this could be related to the scare we saw on Ryuken's mother which does seem to suggest it's a hollow scar. That means the mother was somehow able to survive a hollow attack however Masaki had to go to Urahara for help.
Now here's a thought, it's possible that with the last of her power Uryu's mother did something to protect Uryu from being selected in the same way that Masaki saved Ichigo from Grand Fisher eating him. The reason I say this is because from the story progression we have learned that Uryu/Ichigo in a way follow the same path, they both have gone through similar ordeals and have both dealt with them in semi similar ways.
Both of them used a power sacrificial strike to overcome a foe which they wanted to stop, both regained their powers through their peers, both are stubborn with their ideals and both lost their mother due to Bach's actions.
Uryu did this before Ichigo but Uryu did save Ichigo twice in a sense, first time when he used his quincy ability to release the overexposed reiatsu that Ichigo had gathered and was consuming him and again when he helped to keep Ichigo's humanity when he was on a dead set bloodlust to kill Ulqui. On the otherhand what has Ichigo done for Uryu in context? Again it was Uryu that arrived in time to warn Ichigo that it was Ginjo who attacked him but when you think of things as a whole Ichigo never really "saved" Uryu at all.
Even Chad and Orihime saved Uryu at one point or another, Chad when they were heading to Soul Society and he picked him up to help him make it before being touched by the cleaner and Orhime saved him by using her powers to help soften the fall upon invading SS as well as protecting him from the bomb blast set by Mayuri.
It's rather interesting when you think about it in that sense cause while Ichigo and Uryu have been through quite a bit in the grand scheme of things Ichigo hasn't really done much to "save" Uryu the closest he did to that was destroying the Menos Grande that appeared in KT. Ryuken saved Uryu from a hollow. Even Renji "saved" Uryu through tag team effort even though their foe was more than they could handle.
Though reason I mention all this is mainly related to Ichigo's full hollow form and what Uryu did to save him. I wonder if something similar may happen, since we already saw Uryu attack and discourage Ichigo from coming any closer to Bach but again that could be in an effort to save him since it's very possible Bach wants Ichigo to go up with him but he doesn't want to lead on Ichigo to believe that. The reason he set things as he did was to entice Ichigo to come back up but in a way where the rest of the Gotei 13 woudln't follow. I woudln't be surprised if something does occur in the sense that Ichigo falls into Bach's plan and Uryu expresses frustration over Ichigo not listening to him since he never does.
