So if Chara keeps frisk in that cell for a few more hours, asgore and toriel would appear and free them. So you see, there are many factors in this that ruins their plan every time. Especially since Frisk told Blooky and Mettaton. I have the feeling those two would tell their friends about what happened.
it sucks to be chara except when it doesn’t. this is why chara was in such a rush to return home and expedite their plan with asriel. time was working against them.