ERP systems are great tools to drive increased productivity but it just doesn't magically happens; there are multiple reasons for the users to be unhappy about the system and unhappy people is something that no organization wants nor needs. No examples needed.
Do you have formal business processes in place?
People gets frustrated when they are told to do the same thing in different ways without an explanation or without understanding the logic behind the decision; furthermore they get demotivated needing to wait on their Supervisor for a decision on how to do "X" every time.
This is a clear symptom of lacking formal processes so they are not really unhappy WITH the system but with how they need to "map" a task or activity in the system and needing to wait on somebody for it. Perhaps there is documentation but is outdated thus it is not worth the reading.
We can help you bring happiness back by empowering them with documented and optimized processes that will ensure they do the right thing every time.