Personally, I take constructive feedback and sometimes let it consume my mind for far longer than it should. I immediately react and try to correct my behavior. So it's no surprise that I am always baffled when someone doesn't react the same way. I'm not saying I expect people to let constructive feedback take over mental space in the way I habitually do, but as a manager I think it's reasonable to ask that the employee receives the feedback and works toward making adjustments to improve.
So what do you do when you've given constructive feedback and your employee just doesn't seem to do anything about it?
There are a million avenues you could take, but here are a few that I have found to be successful:
Send a follow up email
Sometimes people don't hear you because they just aren't listening. I've seen a few times where an employee might "shut down" so to speak when they feel like they are receiving negative feedback. If you feel like this might be the case, a follow up email after the face-to-face interaction that clearly states the action or behavior the employee needs to improve can send a very clear message. Some people just need to see things on paper to understand it. Be sure to add in some positivity in there as well. No one wants to receive an email bashing.
Take them out of the office
Whether for coffee or lunch, I have found a lot of success in removing people from the normal office setting to get them to open up about more difficult topics like performance feedback. It can be extremely helpful to put yourself in a more relaxed environment so you can learn more about the why behind an employee's behavior. Hopefully during this time you are also able to find common ground to help them relate and understand the why for the necessity of their behavior change. It's also just a great bonding experience since you are most likely going to chat about topics that are completely unrelated to work!
Get someone else involved
Sometimes it takes a little extra muscle to make people move. I try to use the other techniques before bringing in someone else, but it can be extremely powerful to have your manager involved in a crucial conversation. They can not only reiterate your feedback, but also provide some additional context to why their area of opportunity needs to be addressed. It's also good to just have some backup in the room in case things get weird.
Again, these are just a few ideas that might help you in these situations. Leave a comment sharing what's worked for you when your employee just isn't hearing you!