Probably a timing issue. Its not seeing the previous days backup as actually older than 1 day yet. To get around this can set maint plan to delete older than, say, 23 hours.

Also, I understand you only have room for only 1 days backup, but I would caution you not to delete the only backup you have before creating another backup. If your backup should fail you will now have no backups. Either get more disk space for multiple backups and/or copy your existing backup to another server or tape before deleting.