Perhaps your Playlists without Sequence column are Smartlists or Playlistsgroups?!
You only will be able to see a sequence column for a "normal" Playlists. For those lists you are able to set an order using drag and drop. You add files to a playlist by adding them directly, manually to that list using eg. the "send to" command, the action window or drag and drop.
Selecting a Playlistgroup shows a combination of all files sub-playlists, sub-smartlists and sub-playlistgroups are containing. You can not sort the files shown in a playlistgroup using drag and drop but eg. clicking the column header.
The content of a Smartlist is based on the causative rule. So the content of this list may change based on the content of your library. You are able to change the sorting of this list using logical rules eg. in the smartlisteditor or again by clicking the column headers. Ordering by Drag and Drop is not possible for Smartlists.