The main challenges related to ISO 27001 implementation are:
Lack of management support: without this support, you won't have the minimal resources and engagement to implement the required controls.
Not using a project management approach: such implementation involves coordinating several people to perform dozens of activities, and without a methodology, you will finish inside a huge mess with no security at all.
Lack of time for the implementation project: The project can be very important, but normally, there are a lot of urgent things happening that postpone the project.
ISMS scope wrongly defined: not protecting information that really matters.
Documentation: Procedures excess or lack of details may compromise operations.
This article will provide you additional information: