2Point to check
If you get used to horizontal counting, then you can spread the money quite easily with only one hand, and the other hand is just to support.
One problem you may have is the bill does not spread enough at the end of the fan. Therefore you should check if your hand is not preventing the money to spread. Also, you should not hold the bill too strong with your thumb. As the money spread, you may adjust the position of your hand and that will help the money to spread nicely and evenly.