If someone you work with has dropped the ball on something you owe it to them, yourself, and the organization to be honest about that. Don’t blame or be rude and don’t make it a personal attack. It isn’t about the person. But you have to say something.
Don’t soften it. Every time I have to give critical feedback I’m tempted to cushion it. Too many times I’ve given in to that temptation. “This didn’t get done right but you did great on this!” Many people teach this as the proper way to deliver critical feedback. Adding that “but…” softens it for both you and them.
Don’t soften it. Let the critical feedback hang in the air. It may feel like minutes have passed. Focus on the point you are communicating now and work through it. Don’t distract yourself with the positive things that person has done right this moment. Communicate about that one point. Telling them what they do great will come afterwards.