I had the same problem and i replaced the magnet with magnetic tape aswell. Here is something i noticed:

A magnet only works with the sensor when in a certain angle. Seems
the magnetic poles have to be aligned in a certain way. I had to cut several pieces from diffrent angles so that the sensor would work.

As a summary:
When trying to replace the magnet don't give up if it doesn't seem to work at first. Find the proper angle for the magnet to work with the sensor and then cut accordingly.