Take Iran. The US and several others sanctioned them over trying to develop nuclear weapons. Iran stopped the weapons program, agreed to inspections to verify compliance, and the sanctions were lifted.
Then the US put back sanctions for domestic political reasons, and so now when Iran does something the US does not like (such as supplying Russia with weapons to use against Ukraine), the US doesn't really have a way to deal with that because (1) they've already got Iran nearly maximally sanctioned, and (2) even if they could find something else to sanction Iran over there is no reason for Iran to believe that changing their behavior would lead to removal of that sanction.
[1] https://en.wikipedia.org/wiki/Joint_Comprehensive_Plan_of_Ac...