Always save your DTM before running Generate Inferred breaklines. It has no undo and sometimes the results are not useful. You will want to be able to revert back if that ends up being the case.
Sometimes, a few manually added spot grade points can fix the problem. I have even been known to add breaklines and edit them as needed,
I believe there is a Display Flat Areas tool that can help identify flat spots in a DTM created from contours. As a wise PE once said - "Contours are Output, not Input."
When you build a DTM from contours, there is no easy way for the software to know where ridge and valley lines are needed to avoid the flat areas you see. Inferred breaklines attempts to create these, but can only go so far and can be finicky.