You can remain in the same division for 2 weeks. If at the end of the two weeks you have not accumulated sufficient coins to move up to the next division, you are moved back one division. In your case I would guess that you scored sufficient coins to remain in your current division, but did not accumulate enough coins to advance to the next division. If you look at your standing in the aggregate, you will see three “sub divisions” within your division
1. Advancing up one division (top 30 coin earners)
2. Remaining in your division (30 coin earners who have not accumulated sufficient coins to advance, but have sufficient coin earnings to maintain position)
3. Moving back one division (everybody who is not a 1 or 2 or has been a 2 for a 2 week period)
Advancing in mobile divisions is all based on coin earnings for the week.