1. The platform you are using no longer fits your needs.
When you choose a platform you need to take a lot of factors into consideration... it is always good to review those factors from time to time. What used to work for you may not be working anymore.
2. Your current location isn't mobile optimized.
Let's face it, if you are not mobile friendly you stand the chance of losing followers and not gaining new ones.
3. Support is not in their vocabulary.
This is especially important if you are new to blogging or not a high tech kind of person. You need a platform and/or host that is going to be there when you need help.
4. Your wanting a more personalized site.
Free websites are great when you are just starting out. If you are looking to personalize your theme and make it you; it may be time to look at other options.
5. Your not meeting your objective.
If your current location is not attracting the right followers, you are not gaining the right leads, not achieving your goals or whatever it is your trying to do... then it is time.