Unable to make subfolders in Thunderbird 91.3.0

Am Using Thunderbird 91.3.0 and Widows 10. Have used it for years with few problems. Every year I add a sub folder to the already created folders under the inbox. This is to increase the folder year number by 1 ie if I have a folder marked House 2020, at the beginning of the year I create a new sub folder called House 2021 I have tried to add a new sub folder called House 2022 and for some reason, this year I add it and it doesn't appear in the list. Ive done this for the last 5 years no problem. Suddenly it wont create the sub folder. Any ideas please? Thanks