It might be because you are required to have a minimum number of positions filled in your roster at all time. For instance, you must have at least 2 running backs at all time. So if you only have 2 RBs, then the system will not allow you to drop 1. You can only substitute the player with another RB.
Was this the issue?
![Head Scratch :headscratch:](https://board.marlincrawler.com/Smileys/marlin/headscratch.gif)