@Laurs Hi & welcome to the forum. Not sure why it would be happening but the first port of call is to try & fix it with a Planner Rebuild. If you press Services on your remote & Select the Help tile, you'll see another tile labelled Sky+ Rebuild. Select that & follow the instructions. It takes around 5 minutes so make sure you're not recording anything or about to record anything before you do it. You won't lose any recordings from your Planner when you do this.
@Laurs That's good to know, glad it helped. It's a good tip to remember if you have any similar issues in the future. If you phoned Sky for help that would be the first thing they would ask you to do anyway.