Leveraged index funds borrow money from banks and invest it. For example if you invest $100k in a 3x fund, it will borrow an additional $200k (from banks) and invest it. Each day it will be rebalanced to maintain the 3x leverage.
So if you are holding the leveraged ETF for a month and the index has a positive return, it is still possible for you to have a negative return.
I'm not saying that's how these index funds work, I don't know. Just beware of anything leveraged.