She reacted to it straight away but quickly recovered when Chloe shut the window.
But when she reopened the window Cruella caught the full blast of all 12 counts of the chimes where as the animals reacted and reverted straight away Cruella had more a delay of resistance but that side was fighting a losing battle as she seemed to walk around in a daze. Her shoulder pads popped out sharper, her jewellery was changing as were her gloves. Ella runs out of the building and confronted by her demons. Spots and spots everywhere and Cruella finally wins out.
If the window remained shut, Ella would have lived a long hearty life of wealth and freedom with her new found friends, Leaving Cruella deep inside her prison cell deep in Ella's conscience. But the opposite happens and Ella is left, probably gone from Cruella's mind altogether.

Possibly, thanks for your response.

Because she wasn't so close to it? Chloe's office is right near Big Ben and Cruella happened to never to be that close to it when it go off.
http://s10.invisionfree.com/Nova_Infinity/