On the Touch, depending on the firmware version(as I recall), once you go to the individual collection you should see an option under the Title, on the left side, prefaced by: "by:" The options are: Recently Read, Title, Author, File Size, File Type, and Date Added. Press the option currently displayed and you get a drop down of choices, my guess is you're set to Date Added and added the files in numerical order by title -without any series number appended to the title...
If you use Calibre, and you really should if you don't already for collection management, you can automate the collection or shelf process. You may need to set things up so the Series Name & Series Number are added before the actual title to get the desired sorting order when sorted by Title. There is a great sticky post by PeterT that can be found here:
https://www.mobileread.com/forums/sho...d.php?t=193184
Good luck.