How big are those files that could not be uploaded?
What is your PHP memory limit and max upload size? You can check it under Settings/System (.../index.php/settings/admin/serverinfo), e.g.:
Just tested drag and drop and even can upload 1,5 GB file with PHP limit 1 GB.
This is because of chunking, you can check during the upload if file piece by piece being uploaded to your user folder in: .../nextcloud/data/USERNAME/uploads/web-file-upload-UUID/, e.g. many 10 MB pieces will be collected in 1 files afterwards:
ls -lahr /var/nextcloud/data/gas/uploads/web-file-upload-c51192307f344f0bf6e0170a0d1f7d9a-1639728856821/
total 1.1G
-rw-r--r-- 1 www-data www-data 160K Dec 17 09:27 985661440.ocTransferId1293616986.part
-rw-r--r-- 1 www-data www-data 10M Dec 17 09:27 975175680
-rw-r--r-- 1 www-data www-data 10M Dec 17 09:26 964689920
-rw-r--r-- 1 www-data www-data 10M Dec 17 09:26 954204160
-rw-r--r-- 1 www-data www-data 10M Dec 17 09:26 943718400
-rw-r--r-- 1 www-data www-data 10M Dec 17 09:16 94371840
...