You'll find more advice here as well. You sometimes may have to move buildings around if you still cannot click on the building where Maggie is.
The following is a quote from a Moderator in another EA forum:
The Simpsons: Tapped Out - Maggie Mini game
For the players who are experiencing an issue when they attempt to tap on Maggie while playing the Where's Maggie Mini game, and your tap is not registering.
There are two ways to correct this issue.
1) Make sure you are tapping on the building and not Maggie.
2) If there are Monorail pieces near the building Maggie is hiding on, the tap will not register. If you remove the Monorail pieces, the tap will register.
EA is working on a fix for this.
Meanwhile... Here's my latest video offering on the subject made with the nifty AZ Screen Recorder App for Android Lollipop. As you watch the video you'll see white dots appear on the screen - that's feedback from the app showing where I've tapped.
Please also note that on the tap where I 'Find Maggie' the screen zooms in and centers on the Building, not Maggie. Maggie is over to the upper left from the center of the screen.
And once again I challenge any person in this thread to post a video of themselves attempting to Find Maggie - tapping in the correct spot - and failing to have it work. In the hundreds of complaints I've seen posted in this and other forums not one person has done this.
To make a video screen capture without modifying your device you can use AZ Screen Recorder if you have a compatible device running Android Lollipop.
For everyone else: borrow a device that will record video. If they won't loan it to you the ask them to do it for you and send you the file.
Open a Youtube account - it's free. Upload the video to your account. Find the link to the video. Post the link here.