I line manage someone who is supposed to deputise for me in my absence. They work full time and I am part time so there are times when they are supposed to work independently and they are aware of this. They are fine with managing the "day to day" duties and simple tasks on those days but seem to really struggle with anything beyond that. They are in a relatively senior position ie not a junior level. They are experienced in the skilled industry we work in and they are paid above average for the industry (although not highly paid, we work in not profit sector).
On days when I have been off work and then come back in, I have come to expect that things will have been missed. These are things I specifically ask them to do in my absence, so I am not expecting them to just guess what needs to be done. I will ask them if they have done it and they will respond with a "No, I didn't get to it, sorry". When asked why, reasons range from not having enough time to feeling unwell (but not being off sick) to being out on a long lunch meeting. Essentially excuses are constantly made and I am sick of it. This is not a one-off thing but a pattern. When it does happen I make a point of stating why it's important that these things are picked up in my absence, and I get another apology but it doesn't change then happens again.
We work in a small, non profit organisation with one HR person who essentially manages the HR administration. There is scope for disciplinary processes but I don't want the stress of going down that route. I just want them to improve. I have invested a large proportion of my time and energy over the last few years on this person and there have been considerable improvements but there are still issues.
Can any other line managers please advise if you have handled similar? Managing this person is making me hate being a manager but I don't know how I can change that. I spend 50% of my week on them - I want to be able to focus more on the main part of my job. It is affecting my happiness at work and making me dread it.